Are you starting with round stock or plate? Or are the wheels already cut and you’re just doing the reliefs? If you’re using plate stock just screw the whole plate down at the corners and use tabs. If it’s round stock you could maybe drill the center holes on a lathe, then bolt the middle and double sided tape the rest, ditto if you have nearly complete wheels and just want to cut the reliefs (spokes).
edit: NVM the first parts, I didn’t see that you where making one product from another already existing product. You could also make a fixture like I used here:
Dan