I started with using sofa cushions, bed comforters and pillows ("Those aren't pillows!!" (Steve Martin: Planes, Trains, and Automobiles)).
I would use chairs and such to situate the cushions to get into certain locations. In other words, whatever I could use that was free I would try first. This led me to make some absorber panels using 1/4" Luan plywood (cheap) and 3-1/2" thick x 15x47 Roxul mineral wool, and cheap fabric. I made these as temporary panels, but they worked so well for first/second reflections that I continued to use them for years, and last week I lent them to a friend.
All of this is to say that unless we experiment, we won't know what will make a difference.
Amongst my earliest attempts at using cushions was to simply place them standing up against the back of the stat panel to see what would change. It pretty much completely cancelled the rear wave from the stat panel, kind of the same that would happen if placing absorbers on the front wall behind the speakers.