For a hole that small, I'd perform a quick MacGyver fiberglass rig-job patch, it'll be good as new in no time.
I'd simply get a thin piece of black felt, from a craft store, cut it in a circle to cover your hole.
Get some 5 minute epoxy. This isn't really much different than fiberglassing epoxy - at least for our purposes here.
Squeeze some out, mix it up.
Spread it on both sides of the felt patch so it really permeates it well, soaks it up... and stick the patch on the sub, over the hole. Don't use too much resin, as it'll just add weight to the finished piece... but you do want an effective seal (not to mention bonding) between the cone and the patch.
That patch will not come off by any means, it'll probably be stronger than the cone when it cures. Give it a day to cure before using it. //content.invisioncic.com/y282845/emoticons/cool.gif.3bcaf8f141236c00f8044d07150e34f7.gif