Neither should make a tremendous effect, but you can spot a piece of wood to close the front cutout--just paint it black or cover it with tolex/grillecloth for aesthetic purposes. It should sound like any other open back cab at that point. The back panel may have helped focus the bass, but you'd never be able to tell in a live mix, or when close-micing.I can get a marshall 2x12 combo shell with the head removed for pretty cheap. How does having the amp guts out of the shell affect the sound, if anything?
Also, does a missing back panel really affect the sound too?