It really depends on what mic you have. Is it a cardioid pattern or omni-directional? If it's cardioid it means the sound behind the microphone is barely being recorded so a reflection filter, in a cardioid microphone case, probably isn't the best choice. Hang some blankets or buy acoustic panels, place them in a corner evenly, (How you want it to sound. Test it before you place it on the wall) then place your microphone facing away from the opening of the corner and you WILL hear results!
Hope That Helped!!