John is not talking about a screw thread hole, he's talking about an entire tripod head.
The Grover has a six-sided monorail. It came with a tripod head, and on the top of the head was a clamp that folded over the six-sided monorail to hold the monorail (and thus the camera) to the tripod head.
The only solution I can imagine is taking the monorail to a machine shop, having them take a cube of metal a few inches on a side, drill the bottom for whatever thread your regular tripod head takes, machine out the top of the block so half of the monorail will drop down into the machined out portion, then make a strap to go over the top part of the monorail - again machined to fit the six-sided monorail, and drill and thread a couple of holes to bolt down the strap. No idea what this would cost - you may find another camera with the tripod mount cheaper. I don't think wood would be strong enough given the length of the Grover monorail.
I hope the above description is clear - if you've seen a photograph of the part you're missing, I think you'll u
nderstand what I'm talking about.