Use whatever's at hand that will work. That said, I like plywood for the
bottom and 1/4" baltic birch is darn near perfect, IMO. It's thin and
stays flat. Hardboard gets dinged up easily, but it will work for awhile.
Even so, I have one sled with a 3/4" MDF bed, but it eats up 1/2" more
cutting capacity than the thinner sleds, and this has caused an
occasional problem with thicker stock.
You can use any straight-grained wood for the fences. Even a 2x4, if you
are lucky enough to have a decent one on hand.
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