I just line the hole with split foam tubing or something similar. Protects the cable from damage and closes the hole. What could be simpler.

Regarding the ignition for the G38, did you get the one for 5/8" hex plugs or 3/4"? The one for 5/8" would take a much smaller hole. You could even downsize to a CM -6 size plug with an adapter and the cap would be even smaller.

Worse case .... just remove and replace the plug cap. New ones are not that expensive.