I would think that pine plywood would do just fine.
It is more a matter of thickness, I would imagine.
Aside: Didn't the '50's tweeds use a 'floating' baffle?
" When such a baffle is also less firmly affixed to the front of the cab, such as with just one bolt or screw in each corner, as with the so-called “floating baffles” used in many tweed amps of the 1950s, they really get moving when the amp is cranked and roaring"
Link: Tone Tips: Speaker Cabinets