You can built it out of whatever thickness you want provided you try one of the following designs.
The first design is what I most often recommend. What you do is simply cut a rectangle out of the floor of the cage that is about 2" wider and longer than the size of the heating pad you will require. Keep it away from the walls of the cage as to not run into screws or other fasteners and to maintain the structure of the cage.
Then you cover the entire floor with a sheet of expanded PVC, FRP or just about any plastic than can transmit heat.
The heat pad is the taped to the underside of the plastic floor with the appropriate foil tape. A shallow groove can be routed into the wood so the cords can run out of the back or side of the cage.
The second design is a lot simpler but many are not comfortable with it. I have used it quite a bit and have even seen it used on cages up to 8' long. I have even used it on cages that had several inches of deep soil substrate.
This design is a floor-less design where you build the basic cage carcass without a floor. Instead you just use silicone and staples to attach a thin floor to the underside of the cage. This provides some structure.
I really like this design because it's lighter, cheaper and makes for a floor that is very easy to replace. You can use thin strips of wood to duplicate the air space that is provided in the first design.
-----
Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.