I was always annoyed about the wobbly locking slide on the hopper of the Macap M2M coffee grinder. I never ever needed it so I removed it completely. But now smaller coffee beans and rice grains that I use for cleaning are always escaping through that nasty hole.

Here's the solution. It does nothing else but closing that hole.
Hope it's useful to someone.
Make sure to use a food safe filament. PLA is basically ok, but if you
buy certified food-safe-material you are on the safe side.