Mine are stacked up, so getting pics will be hard to say the least - but I have several AP cages. The bottoms are routed out for heat cable and / or flexwatt (one side is heat cable, the other is flexwatt on mine). The area the flexwatt is on is pretty thin to allow for heat transfer and air circulation around the flexwatt. There is also a spot routed out for the tstat probe.
I imagine you could specify which side you would like the flexwatt to be on - in my experience the gang at AP is very easy to work with and pretty flexible with things to meet your needs.
That said - I use Pro Heat RHP's with my cages and love them... if you have the means to get them, I would highly recommend them.