So, here it is. Dummied up. Not painted yet.
Found some scrap 3/4" plywood. Screwed together two layers.
Laid one crankcase half on the wood. Used a transfer punch to locate the holes.
Poked in some holes. Screwed in four 3/8-16 inserts into the plywood. Used some scrap lengths of 3/8 threaded rod and a few nuts and washers.
The holes go straight through, so the threaded rod can be inserted from either side, making it possible to lay either the left or right case half on the gag, depending on which side you have up. The nut and washer on the bottom stiffen up the rod, the ones on the top set the height needed for whichever half you are using.
Pretty stupid simple, easy to make, just some scrap and a few inserts.
I know, you used a petrified armadillo carcass when you learned this in the Army, blah blah blah.
Just posting this stuff for general interest, and maybe inspiration. Not saying anyone has to do it this way