You can also add a shelf to the AP cages.
I have read that people that just use Flexwatt have no problem keeping the temps right in a room that's in the lower 70s.
Oh. I'm also going with glass just because I feel it will hold up better over time. Not that I'm using a wire brush on the plexiglass or anything :p
I'm not 100% sure but I think there is a spot for the probe on the bottom of the AP cages.