I don't know how they look inside but at least on bicycle wheels the spokes protrude the rim and thus it is not flat.
Here's what you are aiming for, slightly inward-sloping wheel surfaces and a small flange on the inside. I think it could be accomplished with a car steelie rim cut in half, for example.

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Tomas, Sweden
1966 Mercedes Benz 230S with OM617.912, automatic. Disk brakes from W108
1983 Mercedes Benz 300TD grey, OM617.912 and 5-speed manual
1983 Mercedes Benz 300TD blue 7-seater, OM617.912 and 5-speed manual