Are you dead set on having the space behind the background for your filtration?
I agree that it looks much cleaner to have your equipment hidden behind the background but it can pose some issues. I made my 1st DIY background and left a 2" space behind it for the canister filter intakes. I installed three 4"L x 3"H pieces of egg crate (lighting grid) covered with gutter guard in the bottom portion of the background to allow sufficient water to reach the filter intakes. I eventually removed the gutter guard because I was using live plants and they clogged it up with loose or dislodged plants. I also find that fry and juvenile fish find the space behind the background a great place to live and hide but extremely difficult to remove.
I opted to install the next DIY background directly to the rear of the tank but left two empty spaces at either end of the tank background and just painted my intakes to blend into the background. This method is working great and will be the only method I use in the future. It's easy to spot any clogging of the intakes and they aren't really visible if you aren't looking for them.
Also, if you should decide to add any power filters, you might want to consider leaving the height of the background about an inch below the trim to allow clearance for them.
A heater can be installed horizontally, if it's submersible, to hide it better or if you use an inline heater, double check the canister filter output hose size before you buy one. I'm using a Hydor ETH 300W heater on both my 220G and 125G tanks and they work great on the Eheim 2260 output hoses.