This is a handle/bearing cover for the popular Shimano Baitrunner fishing reel. This will fit the following models as well as many others.

Baitrunner 3500, 4500, 6500.

Shimano no longer produces this part so I designed a replacement. I recommend to print this in ASA or a filament that can withstand the salt and sun conditions of offshore fishing. Best using the .12 profile with 4 walls. Will upload more photos soon.