two wraps of a thin tape may work.

i've noticed that the rims i have been using lately (a modest V sectioned rim) have holes that are just about as wide as the narrow tape that is widely (no pun here) used. i have to be very careful not the expose the edge of a hole and careful with the tire irons when mounting a dismounting a tire. and while the distance the tape is forced into the spoke hole when the tire is inflated is miniscule, it seems to be just enough to make it almost impossible to cover the entire whole. i'm talking less than a millimeter off here and you're cooked.

i've found two wraps of johnson and johnson adhesive tape (narrow) can work a little better.