I bought a used Olympus OM-1. The previous owner drilled two holes in the top plate to attach a custom hot shoe with screws. Now I want to remove the hot shoe and the screws that are holding it, and then cover the holes with electrical tape, but I am afraid that it won't be lightproof. How critical for the top late of an slr to be lightproof?
Light entering through the holes above the pentaprism may affect the light meter if it is located within the pentaprism compartment (as MichaelC notes). However, the compartment is isolated from the film compartment, so light entering the holes will not affect film exposure itself, as long as there are no other light leaks in the camera.
Even if the light meter is not affected, light entering the holes may affect your view through the viewfinder, so you may still want to cover them anyway. You can use gaffer tape, which should be sufficiently light tight. You can also screw a metal plate over them, as rackandboneman suggests.