Now that I know about the project................

You can spend your own money, any way you want, but
instead of an expensive machined piece,
what you really need is a 2mm/3mm thick piece of aluminum
that fits the hub within a few thousandths, but does not have to be
"tight-fitting hub centric" since it does not support any weight;
like your "hub-centric" wheels do......and....the spacer
needs to have the stud holes within a few thousandths to the 5 wheel/lug studs, but not hogged-out like the general-purpose spacers, that fit sloppily around the hub and sloppily around the wheel studs.

I frequently make things like this for race cars, but
match and fit them by hand. If interested, get back to me.-Bob