I know of at least 2 people on this forum that have built greenhouses from the portable carports.
But not using the rip stop cover you are talking about.
Have you checked on the life expectancy of it. Most of those type covers only last 2 years. Your's may be different.
Here's some pic's of Patris's greenhouse. I hope she doesn't mind.

Here is a link that might be useful: Greenhouse from carport frame

Don't mind at all Cactus, I rather enjoy showing off our creation!
If I can help in anyway just let me know.

Total cost involved with alum carport frame, wood, electrical, time, sheeting/glazing? I'm intrigued, but want to cost justify this apposed to other designs and prefabbed systems and kits. Thanks.

Kind of hard to do a break down as we did all the work ourselves.

After we were done I think our total was right at $3100.
Greenhouse is 24 x 18 and I have 2 elec. plugs, 7 faucets, 2 ceiling fans, 1 exhaust fan and of course ground cover cloth under the cement blocks.
The carport was $495 bought just before they went up $100, the roof and wall panels are from Lowes high grade for roof and med. grade for walls.
This also includes the benches which the tops were bought from Park Seeds and we made the frames.

I looked at many greenhouses before we decided on this route and I could not find one with the strength or the square footage I have with this one for under $5000
Took a little longer to complete, but in my opinion well worth it.

This does not include the shade cloth which was $145

2 electrical plugs? WOW! I don't know how you do it...

I believe I am looking a total of around $600 for a shelter with the translucent woven poly fabric. The material has UV protection and is suppose to last a good 7 or 8 years. Replacement fabric for the main stretch is suppose to run around $150. This seemed like a good value to me for a green house that could be put up in an afternoon and easily taken down and transported in the future. Of course this doesn't have any of the nice features that Patris has, but it's a start.

You might want to check out the structures made by Coverit, Cover-It, or something like that. Do a Google search. I bought their 12x20 rounded top car cover 6 years ago. Cost was about $450 plus shipping as I recall. Used it three years and a neighbor now has it, still in good condition. It has held up under a three foot snow fall and a couple gale force winds. I think they make a greenhouse cover for their structures.

We are currently using a Portable garage greenhouse. I started a thread on what we are doing right & wrong. We live in Texas so our winters are quite mild. We really only need some protection for about 3 months. We purchased the white enclosure that is sold with the frame, to this we have also purchased an additional clear valance cover and side panel. This is where our greatest light source is from. The quality of the clear cover is very good this guy has been doing this for 30 years! I actually met him he is local-but sell all over. The peak is an issue you need to get that part right so it fits. If anyone could suggest a duct type tape that is locally available it would be of great help. Our heat will just melt anything. Our house comes down in the summer. But the tape is our issue. The corners will need to be sealed as well as around the valance top. We need a tape that will not destroy the plastic when it is removed, but strong enough to take the elements.

They make a different type of portable it looks like you are in the North so snow is an issue. Northern Tool makes a strong frame type shed you might look at it is more expensive than what I purchased but well worth it for your needs. I also found someone that sells the connecting pieces so you could use steel fence posts as supports instead of a pre-made frame
But overall we a quite pleased so far. We collect Brugs & Plumerias they get quite large so the Brugs barely fit. 9.5 inch peak. Next year we will loft the house about a foot. If I have time we might do this year.

Good Luck check out our little house. If you need some links let me know.

Karen

Here is a link that might be useful: Our Greenhouse Project

Big Dogues,
Came across this greenhouse tonight on the net and thought of your post. Perhaps it will help...

Here is a link that might be useful: Carport Greenhouse

I see this is an old post but my mother lives in Whitehorse, Yukon (zone 4a) and has used portable car shelters to grow amazing food for years. I believe hers are "Shelter Logic" 12'x8'x24' and she uses the white poly cover it came with.