I like Dogs plywood idea for some areas of the hull...may be with some neoprene sand witched between it and the boat....but yes a compound curve would be difficult
Still, it might be a good idea, but it doesn't work in practice. Yes, it needs to be secured on the outside of the hull, but there is usually no way (assuming a plastic boat here) you can fasten the plywood in place long enough for the glue to hold.
If you doubt me, try this next time you borrow the neighbour's fibreglass dink.. on a nice day in an anchorage with slight chop:
1. Pre-drill some screws into corners of some scrap ply using your cordless drill.
2. Go swimming (wearing a lifejacket) - don't drop the cordless!
3. Try to drive the screws into the hull of the dinghy - start above water if you like - you'll experience great difficulty in:
(a) Holding the sheet in the right place with one hand whilst holding the cordless above water with the other and,
(b) Penetrating the gelcoat with the screws.
4. Try and work out how you'd get the screws in below water level.
5. Go buy a wooden boat - you can use nails then!!
A better idea:
1. Leave the thin plywood at home and select an appropriately-sized plywood hatch cover from elsewhere on the boat. There's usually quite a range to choose from..
2. Tie a rope to the finger-hole and position it over the outside of the hull. Use a sealant if you like, but you'll only end up with a sticky mess inside the boat as it gets torn off by water pressure before you can get the patch in place. Some Epoxy Putty might work to slow the leak even further, though only over a small hole, but will probably crack off with flexing of the damaged hull.