I've not heard that BRB need to be in a cage of that size!
The AP comes with vents, you wont need any more.
Your getting a baby right? Better think about a different set up while he grows into his new one. They are big cages....
An adult can't open the sliders, get a lock anyways. Acrylic scratches easily.
Get the flexwatt if you want, or use heat cable. Either way, use a good quality programmable thermostat.
I like the white.