Hello mucker, i used to work a sabre 404, which had the normal aluminum base which was covered in 10mm compressed foam board. This foam board is widely available from sign companies, and is sacrificial. It was self routered to form the vacum bed. Hope this helps.

ps, i wouldn't use mdf personally as i feel that it has a mind of its own and distorts too easily especially if it gets wet for some reason, (lubricant for ally?)

cheers alex.