Hi Art,
If you're talking the newer Fender/Sunn 4x10 bass cab, is there a comparable new "Bassman" model made in its image? If there is, a cover for that beast might fit. Sorry I'm not brushed-up on my current Fender line.
For any other cover concerns, you could try Tuki Covers:
http://www.tukicovers.com/I've gotten 3 covers from Tuki, 1 of 'em custom, and they are really nice. You can specify the measurements and features and they'll make a unique cover for you.
Good luck!