You can get a couple small pieces of plywood 3/8"+ thickness. Cut them to fit in the plastic lip on the tank. Make sure to put something heavy on top to prevent escape. The plywood will help keep humidity in so you may want to seal or paint the inner side. If you use a heat lamp just cut a hole and attach some 1/4" steel mesh on top. Hope this helps.

Also according to your measurements it is a 30 breeder.