Ouch! Nasty little C-5's. It's a fairly easy fix, if you want to try it at home. Yank that puppy out, most likely there isn't any fabric over the top, but if there is, grind it away along the edge of the box.
Clean up the inside of the hole and see if there is any loose EPS pellets. If the hole is pretty much the same shape as the box, you're golden! If not, you'll need to see how bad it is. If it's really just a minor mishape, you can fix it when you re-glue the box, if it's bad, you'll want to fill it with foam and quite frankly, that's the time to send it to Mike.
Clean the C-5 box up, a small dremel works great, just get rid of the old epoxy. Test fit it in to the hole to make sure that it fits, easily. If it sits out of whack or too deep, you'll want to fill the hole with a few eps beads - you can snatch them in the insulation aisle of home depot
Get some decent SLOW cure epoxy - west systems is probably available at your local west marine store, that's a good epoxy. They should also have a small batch of cab-o-sil, get that and the slow epoxy. You'll probably need to mix 5 oz just get the mix right, but that will be way more than you need to fix the hole. As you mix the epoxy, pour in the cab until you get a consistency of loose peanut butter.
Fill the hole about 1/2 full of the mixture and grab the box, WITH the fin screwed in, and slide it down into the hole slowly.
You want to get the fin box back into the hole at the same angle, which can take some doing. As you are pushing the box in, you'll most likely squeeze epoxy out. Wipe that up with a damp cloth, being sure that none gets stuck between the fin and board or box.
Once the box is in the slop
, and it's in position, use some masking tape to hold it in place, run a piece on each side of the top of the fin down to the board - it'll sort of look like a teepee. Let it cure for the 4 or so hours and you're golden.
I'd inspect the outline to be sure there are no pinholes and I'd also do the repair in the cool of the evening to avoid the epoxy bubbling as it cures.
It's a pretty easy fix and there really isn't any pressure on the C-5's while riding, so they don't need a fabric cover.
Pour some of the mixture in the hole and