To install a filter behind the background is very easy. Here's a guide to how i do it.

Since i only use the white Bio balls in my aquarium, so the guidelines will only address them as coarse filter.
A filter behind a background consisting of the following: Sinking Bio balls (coarse filter) Filter wool (fine filter) Filter Sponge (used to keep bioboldene and filter-separated, and to prevent the filter wool to wrap themselves in the pump). In addition, used filter sponge also to lay the pump on, so as not to create resonance in the aquarium.

Bio balls shall occupy approximately 60% of the total space behind the back. Since we never should have cleaned the bio balls, it is not so important that we can get her hands down behind the background where bioboldene lies. In picture # 2 is inlet grates in the lower left side, where we also have the bio balls.

 The first thing we must do is to cut the filter sponge. The first piece should sit where the bio balls goes. It is not essential that the filter follows the lines of the background, as it has no significance filtration. However, it is important that it fits properly in the spread between the glass and the background without being compressed. If you push the pores too much together in the sponge, you risk the slammer for more than a few years. Next, the sponge filter, cut to size. Space for filter wool should be approximately 15-20% of the tank full.

 When the sponge is installed, you will come the filter wool in.

 This is very important that you get the filter wool to follow the lines of the background as possible, and ends relatively close up against the glass.

 The harder you squeeze the filter wool together, the better it removes small particles from the water, and the more often it should be changed.

 Bio balls now poured into the chamber. Make sure that the entire chamber is full, since the water flows where there is at least resistance. if you dont fill chamber up, you risk that the filtering is noticeably impaired.

 Cut a piece of filter sponge, as you get to the bottom of the room where the poverhead will lie. Put  the outlet hose on the powerhead, and drop it down. Cut possibly some small pieces of filter sponge and place them between the powerhead and the glass and between the powerhead and the background. 

The outlet hose placed above the background so that it sits in the spread between light and diffuser. That way, you dont need to drill into your background, and can continuously correct the outlet hose so everything bad is fed into inlet grates to the filter. An optional heater can be placed in the room with power overhead.