speakers close to the back wall ?

Talk to your speaker manufacturer first as not all speakers react the same to their ports being filled, but Stanwal has a good suggestion. You can get some closed cell foam and shove it into the port. This will allow you to create a sealed box. Alternately you can try stuffing polyfill or other "looser" material in there.

If another speaker is your preference the I'd suggest the Audiokinesis Rhythm Prisms or Audio Note speakers.