I've seen guys do a pretty nice job making homemade wheel weights with concrete just in case you end up not finding what you're looking for.
Threaded "L" bolts held together with a wooden form in the wheel hole pattern inside a plastic bucket for the concrete form would do a nice job.
Threaded "L" bolts held together with a wooden form in the wheel hole pattern inside a plastic bucket for the concrete form would do a nice job.