This tool started out as a stock EMPI 5775 tool. Basically a big, thick steel washer where the ID just fits the gland nut and the OD is just less than the seal OD. I bought mine from Amazon:
https://www.amazon.com/gp/product/B003LIX3MAI used the gland nut to center the tool on the flywheel and marked one hole, using a 5/16" Harbor Freight center punch.
Next I drilled that hole up to 5/16. Brought the tool back to the firewall and set it up as before, but used my 5/16" drill bit to keep the position of the tool as I marked the rest of the holes:
I recommend going a couple of sizes up from 5/16", like 11/32". I actually used a letter S bit as my 11/32" was out somewhere on the bench.