I am in the midst of restoring a '58 Twin. I have pulled and measured what's left of the original baffle and it was just a hair of 1/4". I was actually quite shocked by that. To replace it, I picked up some 1/4" Baltic Birch. I am a big fan fo light, thin baffles... Literally just enough structure to hold the speakers without failing. I've been using 1/2' cabinat grade plywood for baffles for years. THis'll be my first, really thin, like making me nervous thin, baffle. We'll see what happens! My '59 Bassman Ri is a plywood cabinet. Due to size, and 4 speakers, it really spreads the sound nicely. Very airy and open sounding... Also, not directional at all. My old DRRI was plywood as well. Although, I really don't remember what the baffle was made of... I know Fender's used MDF over the years for a lot of them. **Edit** I have a blank Princeton size baffle all built and ready if anybody needs it (free to whomever). I was gonna install it in my PRRI before I sold it. It had a 12" speaker, and I was thinking of going back to a 10"... But, never got around to it.