I would make an access panel for anything you place under the false bottom or you will regret it if you have a freak failure in a few months.

Yeah in the paint section at walmart by the silicon is where they keep the spray foam. They have the normal Great Stuff, but also have GE brand spray foam. I used it for the first time on my sons Dart tank last week and it worked out really good. No noticeable difference between it and the Great Stuff except the price.

For my filter and Pump I built a channel using Egg crate up the back of the tank so I could access them behind the back wall and remove or replace them. I am not sure how practical that will be, but at least I tried lol.