Why don't you just ask the folks at AP if you can have your cage routed for flex-watt? They've easily accommodated custom requests that I've made...although I do use their heat cable instead of flex-watt.
I'm pretty sure they offer heat tape as an option, so they're probably familiar with the process of outfitting a cage with it.
I have changed my BP's cage over from the AP heat cable to just a Radiant Heat Panel and it has worked perfectly. Temps stay dead on and humidity is NEVER an issue. The RHP does cost a little more but over the long haul I think it is the best option. I have two T-10's (one thanks to KLG) and will probably purchase a third for my Kingsnake. I was using the heat cable and a CHE on the BP cage and I HATED it. The CHE is brutal on humidity and I have run into defective heat cables in the past, plus it is a pain in the butt replacing the cable in the routered channel at the bottom of the cage. It is tough to get it to sit in there nice and flat without popping back out.
0.0.1 Ball Python (Austin)
0.0.1 Albino California King
0.0.1 Corn Snake (Cornsuela)
0.1.0 Doberman/Shepherd/Lab mix (Zora)