I've had that situation. Had 12" in bass bin with Fostex 108ez's on top on ob panel. Rear wave was too strong so tried draping different thicknesses of material over the back pf ob. T-shirt, towel, blanket, etc. Also tried 2" rock wool panels, and 5" bass trap on wall. It can work ok but imo not as good as being 4' out from wall. There's more spaciousness, openness and clarity without damping behind.
Diffusion behind doesn't dull the sound like absorption. Worth trying that too.
But hope you try it anyway as you will get the idea and may like it. Especially if it's just a matter of cutting a hole in some plywood, and a cap bundle (50-100uF or so) on the fr. Maybe you can pull them out at some point to hear them that way too.