To get a perfect fit line to sand or cut to,  put the skeg in place and put a pencil or a blade onto a block that elevates it just a hair above the widest part of the gap. Then pull the block and marker along the hull. It will mark the exact profile of the bottom onto the skeg. A blade will make a more accurate mark.

As a practical matter, if you're using a glass taped filleted joint, a 1/8" (3 mm) gap fully packed with epoxy woodflour putty will be plenty strong.

Laszlo

   Thanks Lazlo.  Sorry to be dense but what is a glass taped joint?  I do t see where fiberglass is used in attaching the keg, if that is what you are referring to.  Bruce

I mean a joint that has a thickened epoxy fillet with glass tape on the fillet. The PMD uses screws and a plain fillet with no glass tape, right? You can also attach the skeg for a boat that size by bedding in thickened epoxy, forming fillets from the squeeze-out (and adding enough extra to make a nice thick fillet and then applying glass tape on the fillet. That style of joint is perfectly happy with a gap of up to 1/8". The putty fills the gap and the glass rides over the putty. The stiffness and strength of that kind of joint is mainly in the epoxied glass tape.

But if you use that marking tip and trime carefully, there won't be a gap and you can use the screws and fillet-only method just fine.

Laszlo