The Ploopy Classic is my all-time favorite mouse/trackball input device. But it does have one fatal flaw: the scroll wheel. Perhaps like myself, you have disabled it and only use drag scrolling - in which case, the big wheel sitting there can be kind of pointless, plus the rubber slides around and is annoying. Eventually mine broke, so I decided to shape a new button.

It installs using the one of the original guide dowels for the wheel, and snaps into the top of the mouse body like the wheel did.

I have not tried printing this on FDM, and the tolerances for the model are very exact - so if you can, I do recommend printing with resin. If not, you may need to mess with the model a bit to get the right fit. I used Anycubic Toughresin 2.0 for the button (and outer shell you see pictured), it seems to work pretty well - but I recommend curing it in warm water to avoid warping.