The rings are not really necessary, if you have the blueprints you can align the ribs by hand. But the alignment rings are much 'cleaner' of an install. . Here you see both the upper and lower rings with the 108 slots cut in them. This will keep both the depth and spacing of the ribs consistent. I also use a piece of 6 inch sewer pipe as the support for the collar ring.
Here is a piece of 1/2 acrlyic mounted to an indexer. This allows for an even 108 slots to be cut.
This is the finished product. You can see each slot and how the rib hits the back edge. I've also milled and inner ring on the reverse side so I can position the support pipe and keep the top in alignment with the bottom. The four holes are from the mounting bolts to hold the plastic to the indexer.
Here's both bottom and top plates.
This actually takes quite a bit of time, the setup is not really that bad. But you have 216 slots to cut. I put in about 6 hours for both plates and machine setup.
$38 in acrylic and then some glue.