FYI, it -is- possible to mod a rubbermaid to use a heat lamp without melting...
This took a soldering iron, 1/4" hardware cloth, a drill, and some nuts & bolts. Soldering iron cut out the original square, cut hardware cloth to fit and set it on top, drill holes in the squares in the hardware cloth where you put the bolts through, fasten with nuts.
The intent with this wasn't really for a heat lamp, but it would work. You wouldn't need as high a wattage as on a glass tank because the plastic will hold heat better. I think I tried this because the other identical tub was having problems with condensation with just holes melted in the lid, so I wanted better ventilation without holes all over the sides.
Also, I like to use human heat pads for rubbermaid containers. They don't get as hot as the zoomed heat pads I have.