To make my own bulkheads and centering rings, I use 1/4" plywood for stock and used a bandsaw to cut the rough circles.
After the blanks were cut, I drilled a ¼" diameter hole in the center and put a 1/4" bolt thorough them. Then I placed the bolt in the drill press and use a grinder to make them perfectly circular and more importantly, the correct size to fit into the couplers. One piece, which is actually flooring laminate, I ground the O.D. to match the O.D. of the coupler for the avionics bay on my project. For the anti-zipper coupler, I installed a 7/8" diameter forstner bit into the drill press and cut 3 holes in each bulkhead.
To convert the bulkheads into centering rings, you need a fly cutter on the drill press. I used some small brads (3 of them) to secure the bulkhead to a piece of base material. Once you have adjusted the fly cutter to the correct diameter, you can clamp the board with the bulkhead attached under the drill press. Just align the hole in the bulkhead with the bit on the fly cutter and carefully cut the center out. I made 3.9" x 54mm centering rings with this method.
Submitted by Chip Jenkins