I use discarded glass bathroom shower doors for my cold frame. They are quite sturdy. And I add a light bulb for extra heat.

Perhaps you could increase the insulation effect of the plastic by applying it double with an airspace between just like double glazed windows for house.

I did something similar with small bags of peatmoss last year for potatoes. Peat moss has been recommended over soil for container gardening so this seemed like a good idea. The yield was not as much as I expected. Of course peat moss needs fertilizer which I used (manure tea). This peat moss bag idea works well for strawberries. I made two large peat moss bags into typical strawberry pots by standing them on end and cutting several slices into the corners and scooping out enough peat moss that I could then push back the plastic above the slice .This indentation created a pouch so planting the strawberries was easy. I then covered them with t-shirt material in a shade of terra cotta. I got this idea from a nursery on the west coast that had a wall of peat moss bags held in place by wire fencing. It was planted with petunias and by summer was a solid wall of blooms. You can do something similar with a stack of old tires too. Cut away some of the tire so there is more planting area exposed. I found that idea by googling and will try it this year. You can build quite a high wall for a vertical garden. We have no soil except hardpan where I am in this trailer park made out of a backfilled garbage dump.
I think this year I will try putting small bags of soil( I bought some at 87cents at the end of last years season) on top of bags of peat moss so the height is where I like it. I want to also create a raised bed for raspberries so will insulate bags of peat moss (plus some soil mixed in) with rigid foam insulation sheets to protect the roots as we get real winter here. I will fence this in with something like a closely spaced picket fence for esthetics. Keep thinking outside the box. I really groove on these ideas!