Cobbling together a mini green house sounds like a great way to go. And I like do-it-yourself stuff, but how many plants are we talking about here? There are some very effective and inexpensive green houses available out there that could accomodate at least a dozen plants for under $20. Here is a link to the one I used last year and will use again this spring when temps rise a bit. Here is a photo of mine from last spring. In the bottom you will notice the small electric heater which has a built in thermostat. I had it set to keep the temp at about 50 or above so it only ran when there was no sun hitting the green house. In the basement, it wouldn't run much at all with the temps being warm already but it would keep them up to where you need them. Those type of green houses are sold at a lot of different places. Check the link below the photo for more info.

Here is a link that might be useful: Mini-Greenhouse

Bruce ..That is a nice one. It can serve more than one purpose ; Indoor, Outdoor, grow, germinate,
$19.90 ? that sounds like a great deal. But you have to watch out for the winds !

Menards in Mpls had a similar one on sale for $15 a few weeks ago. For that price, I should have picked one up. Only trouble I see is they would not work inside when you need to keep lights on 100+ seedlings.

You might try a heat pad to set your trays on.

Here's a link to the one I'm using. I love it, seeds germinated super fast. Order whatever size you want.

Here is a link that might be useful: Heat mat

Yes, you do need to worry about the wind knocking it over. After this photo, I had wrapped a couple bungee cords around it and anchored it to the deck railing next to it. Problem solved.
Lighting in that in a basement might be a problem if you had a bunch of them that need light..

Either go w/ something in Bruce's example or make one. Either way some heatlamps will do the trick on warm. Heck build you a box a/ wrap/cover it with bubble wrap. Water everything down a/ put heat lamp on top of it. ~760lumens should keep it 80.

Some of these sound like fire hazards. I have the greenhouse in the pic... Actually 2. Big lots was cheapest but no wheels, and one from lowes with wheels. Love em. I moved one inside during the current windy storm in CA. I use two seedling heat pads on two shelves for mild heat... With the plastic closed it keeps it warm. Also I can fill it with steamy humidity for grafted tomato recovery.

Hmmm, heat mats. I have two 20x20" heat mats with a thermostat. Do you think those would be enough to keep the whole green house warm at night? I like to get mine set up and going in early April but the average night time lows are the upper 20's early in April and upper 30's by the end of April. The fan heater that I have does the job, but that pulls a lot more power than a couple of 45W heat matts would pull. I realize it would help to have the plants directly on top of the mats but I won't have enough heat mats for all my plants. I just have the two and don't really want to invest in more. I was hoping the matts would be enough to keep the ambiant temperature up to about 50 degrees at night when it could be getting down into the teens possibly. Anyone have any thoughts?
Bruce

Go to Lowes a/or Home Depot a/ get two of the old clamp style lighting ballast. Get two 500W heat bulbs a/ use that.

Inconjunction w/ above put heatmats on middle shelf, 1 facing up a/ 1 facing down to evenly dispose heat produced.

Between the two you should be able to maintain solid heat at night.

jtight: I have the small fan heater already. I was trying to figure a way to heat it without pushing 1000W. I will probably just stick with the heater since it does the job and is thermostatically controlled so I don't have to worry about shutting it off during the day or times it isn't needed.
Bruce

Brian6464 I like your idea about building your own frame. You'll get the dimensions you need and won't need to spend much money. Maybe line some of inside with reflectix. If it isn't too leaky you might find the lights give off enough heat, if not maybe buy more lights. You'll probably want to ventilate it every day. Good Luck.

I've got that same greenhouse. I lined the inside with Mylar emergency blankets one season, and put a chicken brooding lamp in it with a small fan. It kept soil temps in the upper eighties, as I recall. Worked great, probably used an irresponsible amount of electricity though.

If you use heat mats in one of those, you may want to line the shelf it's sitting on with several layers of Mylar to direct the heat up.

I've decided to just build one which I can do for under $10. I ripped some scrap cedar to 1x2x24 inches to square the ends. I'll attach the two ends with 1x2x66 inch cedar strips. I plan to teepee the top at about a 35 to 40 degree angle. Then I'll just staple clear plastic (the only thing I need to purchase) around the frame. These dimensions are big enough to let me just drop it over my 4 foot 6 bulb light. It will fit on my 6 foot table and leave enough room for me to use my small portable heater inside of it as necessary.

While it will not be tall like the picture above, it will serve an indoor basement purpose without needing additional lights or heat mats. The apex of the teepee should be close to 3 feet, so I should be able to use it outside on the patio for small pots early in the spring.

I'll post pics once I get around to finishing it in a couple of days.

Brian:
That sounds like a good idea but I would suggest one possible change to the plans. As long as it is going to be an indoor shelter and you don't have to worry about rain an other elements, then I would make it more square. The reason I say that is I built a similar structure for the top of my above ground garden. One problem that I had with it was that the angled top tended to impede my plants as they got bigger. As long as it is an indoor shelter, I would make it more box like. It will make the construction and covering much easier as well. If you are only going to use it for small seedlings, then that is fine, but if you have bigger plants in there, you might want to square off the top. Here is an old photo of the cover on my above ground garden.