I quickly designed this to plug a hole made by a 6mm drill bit in a concrete wall.

When printed in the same color as the wall, it hides the hole quite well.

The hole must be at least 15mm deep.

I might upload multiple dimensions at a later date. For now, you should be able to scale it slightly in the slicer to fit your needs.

Update:

I’ve added a version with thicker walls for better slicing with a 0.4 mm nozzle.