I always wanted one, but the pain of creating a square hole through 40 plus mm stopped me from making one.
The plastic block gave me an idea. I glued up enough number plates to make up 2/3 of the block, then machines the slots for the punches to fit. Then simply glued another 2 layers of number plate and a square hole with minimal effort. I also milled out a piece below the punches that I could use to line up to a scribed line if I have to do more than 2 characters. I made it to do 2 letters at a time as I mostly stamp 2 letters at a time. Lots of bits making up the suspension bits of a race car. I scribe a line on the side of the holder 0.5mm less than the punches, this gives me a nice reference it I need more than 2 letters and the letters are nice and close.