RE: CARF F4U-1 Corsair build thread
This front ring you will want to taper from front to back, leaving a small flat in the front to ensure that the diameter does not change.
The back ring will be 3/16" smaller in diameter. This allows room for the pivot of the hinge, and ensure that the flaps will be flush with the cowl when closed. It is also tapered but in the opposite direction to form a V groove in the two pieces of ply. The back taper is not critical like the front one is.
A disc sander is a handy tool for the taper, and also is handy for the final shaping of the rings, but a sanding block with some 100 grit also works just fine.