You will need sealed mirror plugs if you want to cover the mirror hole or replace the side mirrors with aftermarket. These mirror plugs bolt to the existing fixing points and are sealed like the original ones to prevent rain water go into the door.
They are available with 7 mm center hole to allow mounting aftermarket side mirrors. Please keep in mind, they are not universal adapters for every aftermarket mirror and modifications to them might be needed. The ones offered here are polished stainless steel.
No drilling is needed for bolting them, as existing securing points are used.
As some early MX5 came without el. mirrors, they did not have a hole for such in the door. In this case you will need to drill the door, just like the later MX5's, to mount your mirrors