Front wall between the speakers one would be more inclined to use diffusion as opposed to absorption, but given your limitations, anything will be better than nothing.
A blanket as suggested is going to work over too limited a frequency range. Heavy pleated curtains may work well as the first wave from the speakers will be at a sharp angle and hit multiple pleats.
A blanket as suggested is going to work over too limited a frequency range. Heavy pleated curtains may work well as the first wave from the speakers will be at a sharp angle and hit multiple pleats.