Use something no heavier than the cone itself, if it's an 1/8" diameter hole. Start by auditioning old-time grocery bags or lunch sacks. Or... got any old crappy speakers around? Harvest from them. Keep the patch pretty small, to preserve that kickin' hi-fi response.
Try to fold down the cone material from the front, and put your patch on the back. If you're lucky, all the pieces will fit and leave little to show. The 'artists pack' of sharpies should have the proper black/gray/brown to complete the masking... plus it's behind a grille, right? Not fully visible like a hi-fi speaker.
Back in the 're-coning JBL' days, you always had to hold the tip of the screwdriver to prevent the magnet from grabbing it. Holding tip of soldering iron was more of a challenge.
Good luck!
Chip