My 6X9 flange is cupped so a bit trickier to fabricate. It's attached to a 120mm Heliar if that make a difference in construction.
Yes, but I don't think you need to cup it. There are 3 bayonet-plugs on the Bergheils (but only 2 on the Zeiss/Ica cameras), which should make it possible to center the ring without the cupping? Any alu-quality should be strong enough to hold these small lenses, except if you're a very hard user? The front plate on the Bergheil is just plain flat. The 120 Heliar is in a Compur no. 1 shutter with 39mm mounting flange, which makes it the larger of the two sizes for the 6x9 Bergheil. The smaller ring is for an old no. 0-compur with 30 mm flange (for Heliar 10,5 f:4,5 and possibly other lenses). But the shutter size don't mean anything in principle - just make the ring plain flat. Have fun.
Update: Sorry, forgot to mention, you have to make a washer (ring) from cardboard or cloth or the like to glue to the underside of the bayonet-ring for lighttight fit.