A cap to cover an 8M nut and rod, where the rod is sticking out no more than 3mm).

I used to cover nut and sharp rod ending, so that you don't accidentally cut yourself on the rod.
Milling the sharp edge down with a file might be a better option in some scenarios.

The cap is intended to be press-fitted with a hammer, once attached it's pretty solid and cannot be removed and reattached.

Once sliced, check that your slicer created the small internal fins, otherwise it won't attach.