What I always recommend is to cut a rectangle out of each floor that is about 2" wider and longer than the UTH source you'll be using. Make this cutout away from the walls of the cage so the structure of the cage stays intact.
Then go ahead and cover the entire floor with Sintra or another type of expanded PVC sign board.
Then just use aluminum foil tape to attach your heat source to the underside of the expanded PVC.
Also route a small groove for the cord to run out the back of the cage.
If the floor of one cage will be acting as the ceiling of another, you might want to take some measure to keep snakes out of there. Perhaps anther piece of Sintra screwed over the opening. This piece would not need to cover the entire ceiling.
Fiberglass reinforced panel (aka FRP board) would also work for the floor.
This design works very well with the Sintra as it increases thermal mass and helps spread out the heat. It also makes the floor of the cage much more durable and easier to clean.