I found an electric motor heat sink that has the same size opening as my Morrow coaster brake shell. I cut it in half, trimmed it from 50 mm wide to 36 mm and filed a grove for the retainer to sit in.
My storage shed has arrived. Interesting how they put it on my gravel pad. Getting gravel delivered out here was what slowed down the progress. Today I’ll start filling it up. A lot more room to hold junk.