I haven’t made one myself. I was under a deadline and was going to make one myself since boxcar was sold out of my size. But i was lucky enough to find a real base on ebay.
Boxcar told me to use cherry or furniture grade birch (or something similar). You need a hard wood that won’t bend, break, warp or get an impression with the pressure of the press.
For my press, with a 5×8 chase, I would have cut the wood down to 3.5″x6.5″
Planed down the wood to .875″, that way when you add your plate (k 152), everything will be type high!
Good luck and happy printing 🙂