Is the hole an un-used cable entry? You should be able to source a plug easily enough. Measure the diameter of the hole and do a search for "Blind Grommet" "blind gland" on ebay or similar.
You dcould even fit a normal cable gland with a short length of plastic in place of cable
Is the hole threaded? Something like this may suit: https://www.tlc-direct.co.uk/Products/EK20BP.html
Anon
February 2016