I wouldn't use plywood anywhere, as it will drastically reduce the effectiveness of the trap. The way a broadband absorber like this works is by absorbing sound energy as it passes through the absorber. If you put plywood anywhere, front or back, you are blocking sound from passing through it.
Just put the 703 straddling the corner, and cover it with cloth. If you need to, build a simple wood frame out of 1x4s or whatever to hold it in place. There are several DIY designs out there.
