I set a cedar table on fire with a water bed heater trying to use it as a propagation mat. Wouldn't have been so bad if it wasn't in my garage and 2 AM. Now you know why I am such a stickler for spending a few extra bucks and not cutting corners.

There will be a few people who say they have done it for years. Yeah, whatever. All it takes is that 1:1000 and your house/ GH is gone. NOT worth it.

Nice tables...

Waterbed heaters are about 350w and designed for going under a water bed bladder. As weebus says, don't even try it.

Propogation mats. Well a lot of them don't have any thermostatic control either, they warn you to just put a flat of dirt over them and nothing else. No cover, nothing.....

You have a nice bench, play it safe. As you have built it then run some plastic pipes under the gravel and use a small pond pump for circulation and an aquarium heater in the holding tank.

You will have much better temperature control, water is safe and aquarium heaters (the submersible kind) will fail safe if your pond pump is above them in the holding tank.

What about heating cables? Also I provided a link to mats that have temp control. Not the cheapest, but probably the safest...

Here is a link that might be useful: Propagation mat

The plastic coated shelving looks like salvaged frig or freezer shelving. Did you find abunch of it in one place or just patiently accumulate it piece by piece.

BTW I'm also in CA, sorta coastal Sonoma County. How Far Norhtern Ca coastal are you?

Thank you for the great suggestions, thoughts, and link. I'm leaning toward the propagation mat idea.
weebus-- I tried some old heating cables, but they don't have the punch to heat the gravel I've got in the boxes. I could eliminate the gravel though, so we might try that. I like the greater precision of temp control with a mat, so we will probably try that.
clavero--the shelving is salvaged frig shelving, collected in one place (amazing what people throw out!). We are in northern Humboldt county about 30 miles north of Eureka, and right on the coast (I can see and hear the waves from here.) Jim

I use an electric blanket without trouble. The blanket wire is double insulated all the way to the controller. The wire sits in a frame on a layer of perlite underneath and coarse sand on top and weld-mesh all around for drainage. The weldmesh is connected directly to earth, so the ELD will trip if the wire gets damaged.

I have used a waterbed heater placed in a bed of sand, then I sit the plants on top of the sand and cover them all with clear plastic. My husband is an electrician and feels the sand solves the fire issues.

PQ