I found this useful, but there's a discrepancy. 2 1/4 converts to 57.1mm. I measured it at 50mm on the dot. So I bought some 51.3mm and it's a super tight fit. Not sure if it's an EU vs US difference or just different manufacturer for the wing.
I also took apart the connector for the brake light so I don't have to poke an oversized hole for better and snug fit to ensure nothing gets in.