Dull question, but why not change to a standard shelf with no silly speakers in it?
Cause that's effort, and would have to buy new speakers for the back with money I don't possess (or wouldn't have sold the nice big focus with the dog box in the boot!). And then I'd probably still want some sort of cover I can take off and clean to keep it looking reasonably tidy.
I think just putting a cover over the seat is the easiest and cheapest option tbh, that's why I wondered if anyone knew of any that fit the mk2 well.