The rear seal plate is one of the last parts to be made from my 125mm diameter by 300mm piece of Aluminum 7075-T6. I bought a stick of this long enough to make the front cover, crankcase, rear seal plate, air guide, impeller, and a few fixtures. The diameter was needed to accommodate the crankcase so I did end up wasting a little material on the other parts.
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Hodgson Part 090, Rear Seal Plate Spacers
I had some scrap Aluminum 7075-T6 left over from some other parts so I knocked off the corners until I could fit it into a collet. I then rough turned it down until is was round.