Dunno if they still do, but my local Home Depot used to sell flat metal stakes that would work nicely, if you can cut and drill one. About 3" wide by 1/4" thick by maybe 3' long with a tapered end. Intended for holding concrete forms, I believe (has staggered nail holes along the length).
Another possiblity might be a cast iron plumbing "floor flange" (intended to locate pipes to a floor where they pass through).
Or yet another might be to section an old TR brake rotor (if you have one of those lying around).