They should be more than adequate; I know someone who kept a male boa in it for a while. I use them as transport tubs for my retics. I would put two large UTH's under it, one to create the hot spot, one at about 82*F, instead of trying to alter the lid to handle an overhead heat source.

Also the lid is very long and it will flex so I would find a way to secure it to the tub halfway down each long side. Otherwise your snake may push its way out.

If you want a slightly smaller tub that will still be adequate, Lowes and Home Depot carry Hefty and Sterilite tubs in the 100-160 quart range that would suffice.