They will need to be vented some way, such as by propping up one side with a small stick. Otherwise, the sun will bake whatever is under them. They won't crack as long as they don't get moist inside, so propping will prevent that. You could turn those aquariums into wonderful propagating chambers with a layer of good potting soil, and a layer of sand. Moisten the contents, stick your cuttings, put the hood on, place a light over it, and be sure to vent it. Things root wonderfully well in this set-up. I've used this method for years with great success.

The sun will bake the plants even in winter?????That cutting propogator sounds like a good idea but I rarely root cuttings.....I am an abysmal failure at rooting cuttings..... :) I could prop the containers up on stones..... :)

Aquariums have a way of collecting ice and cracking. Wrapping shrubs in blankets and plastic tarps is effective. A double layer with gallon plastic jugs of water at the base is good for 8 or 10 degrees moderation. I do a lot of seed and cutting propagation. For cuttings, I use perlite as a medium. I use poly tents over flats and shaded coldframe for cuttings. I've also used a mist system. I have 6 coldframes which I use for various purposes. I find pits are indispensable for seeds, cuttings, coddling of potted palms, etc. Seedlings started in a sunny frame in late February or march are head and shoulders above the weak, spindly windowsill or fluorescent light seedlings. Actually they're pits which I can heat with pipe cables or soil heating cables. I think every gardener should build a frame or pit, even a small 3' by 4' wood box with sashes, poly, or lexan on top.