i just finished a similar skeg installation (different skeg) but almost the same dimensions.

i have done this a couple times and the technique i use is this:

on the outside of the boat i use blue tape to clearly mark the end result of the skeg cutout i am trying to acheive.

so this would be a rectangle 1/2 inch by 15 inches.  i would drill two 1/2 inch holes on either side and carefully, slowly cut with a fine handsaw my line between the two holes....using the blue tape as my guide.   you can use a jig saw....but i find a fine bladed hand saw gives me more control and less chance of a bad error happening.

if you want to finish it with a file to ensure you leave a bit of space, you can drill15/32 hole and move your tape in 1/64 from each side. and then file out to your final opening.   the only other thing i would point out is to make sure if you want only a 15 inch opening, your 1/2 inch hole on each side actually needs to be inset 1/4 inch from each end (so that the hole when drilled will touch the end with the appropriate length.

make sure to wear a dust mask and gloves when doing this work....fibreglass dust is kind of nasty.

i hope this helps.....not a big deal to do this...but you are right to sort it out and get it right....hard to fill an improperly made hole.

h

I have installed skegs in three boats, using the technique outlined by hspira.  Using the 1/2" drill bits at both the forward and aft end of the slot will result in a nice radiused look at both ends.  Since large diameter drill bits can walk a bit when they first dig in, I use a small bit to drill a pilot hole then drill with progressively larger bits until reaching the final diameter.

I also prefer a hand saw for this delicate work.  It is too easy to make a large mistake quickly with a power saw.

From the link that you posted, it appears that you are installing the KayakSport skeg kit with flange.  That kit will work well is the bottom of the kayak is flat.  If not you may want to go with the kit without flange.  That is what I used on two of my boats.

   Hspira and MarkN, thank you both for your help. I like the handsaw idea. The only access I have to the interior of the kayak is through the cockpit and aft access is very limited. I have a closed cell foam block fiberglassed in just behind the cockpit. I put it in after I cut the fiberglass seat out (I did not like the integrated seat). I will not be able to fiberglass the skeg sleeve on the inside so I thougth the flange would add structural integrity to the installed kit. The bottom of the kayak is pretty flat. The flange actually angles up slightly (a small surprise when the kit arrived) so I will use thickened epoxy to fill that gap and glue it in place. I will taper thickend epoxy in front of and behind the slightly raised insert. It will be difficult but I believe I will be able to position predrilled pieces of wood inside the kayak. These will be at either end of the sleeve and screws through the flange (the kit has predrilled holes fore and aft on the flange) will hold them in place. I plan to cut 'U' chanels in the wood, the 'U' will face inward and line up with the hole I cut in the boat so when I slide the insert in it will slide into the fore and aft 'U' slots. This will add strength to the skeg sleeve. I have figured out how to glue these wood pieces in before I tighten the screws. All of this will be done on a plywood mockup before I cut into the boat. Thanks again, I plan to start the first or second week in December. I will let you know how it goes, I will move slowly. Mike 

Rather than all the hastle of working through the cockpit, I'd add an aft hatch.  This will give you access to install the skeg and when you are done will add storage capability.  If the aft deck is relatively flat, I would go with the oval Sea-Lect hatches that CLC sells.  That is the hatch in my picture above that was standard with the PP kit.

   Thanks Mark N the deck is not flat but I will look at options. Mike