My primary stage cab is an odd little 1x12 half-back I scored at a used gear store about 10-12 years ago. It's a Tubeworks cab but it came with a "Mesa Black Shadow" EVM-12L speaker (I now use a JBL E-120)
A couple of years ago, in the open part of the back, I wedged a 18x18 square of a piece of material that had been part of the packing material from a shipped Bag End replacement speaker: a 1" thick soft-fiber (wool?) cab insulation material, apparently what Bag End uses to line the inside of their cabinets. Anyway, it sits wedged in there nicely and helps keeps moisture and dust etc from entering the back of the cab, but it still sounds like an open back speaker.
"There, in huge black letters, was 'The Grateful Dead'. It just... cancelled my mind out."-Garcia