Mikuni SBN carburetor diaphragm cover plates. These are machined from 6061 aluminium for superior corrosion resistance.
The vertical hole in the bottom also removes the issue of your diaphragm hole being blocked by your exhaust if the carbs are touching it due to you running a reed spacer. The breather hole has been relocated to the bottom of the plate to ensure if you do sink your ski any water that gets in there will also drain out.
These will fit 38mm 44mm 46mm carbs as well as all other carbs based of a SBN Mikuni platform. It will also fit your standard Yamaha super-jet carbs as well as a lot of the Kawasaki and sea-doo skis.