Thanks all for the advice. I ended up using a 4-in-hand file to vertical-ize the cockpit cutout, and sizing the strips is now significantly easier. In addition, I set up a sanding block to fine tune my angles (actually, just put the sandpaper on the "palm" side and rested it on the deck). I think even with perfect fit the join on the recess strips is going to be a bit wobbly; perils of a hand-cut circle on an irregular tube shape.

Next time around a thin transition strip around the cockpit would fair this line out on both the deck and recess sides, I think. Until the next boat, then, I'll add this to the list of imperfections that bug me which nobody else will ever notice.

: Good tip - especially for sanding the inside of the kayak.

: I used a 'knock off' of a japanese double-edge saw for my cutout -
: very flexible, and easy to keep a vertical cut, even around the
: tight curves (I curve the blade lengthwise for those cuts - it
: doesn't curve perpendicularly).