we've had summers without ac and the snakes have coped about as well as we have
I've given them refrigerated moss hides on really hot days; just make two plastic food storage container with damp moss and a hole somewhere for each tank and you can trade them in and out of the fridge or freezer; the snakes usually figure them out pretty fast; first they just lie next to them, later they move in; if you insulate the tank a bit, the cool stays longer; I tape those blue foam insulation tiles from the frozen rodent shipments around the tanks
frankly - we only got ac when I used to have tropical fish and realized that for the price of a couple aquarium chillers we could all be more comfortable, but now that there are no more fish, we hardly use it
rhea "you cannot depend on your eyes when your imagination is out of focus" Mark Twain
I did the same thing recently with my 35 gallon Sterilite tub. One thing that I did to keep the lid more secure was drill two holes in the back corners where the lid meets the tub and put in a couple of nuts and bolts. It keeps the back completely secure and I just use a couple of small plastic clamps on the front end. It does make opening the lid a little difficult as it now doesn't open all the way, obviously.