Paul wrote:

The other thing I have considered is make a lost wax casting of a
conformal aluminum
filler block. No NC mill profiling required. Use an epoxy casting as a
pattern.
Machine screw it down with RTV under it to the outer rotor housing. If
it is tall
enough that is  all you need. No O-ring and easy to reseal. This would
be the
most reliable way of doing it. No room for error.

Much better, could be a very sellable item with possible volume potential
if the rotor housings exteriors are reasonably consistent.

Fred O.