Seems like no one is offering a response.

I am unaware of any published plans, but creating a pattern should not be too difficult.

Do an initial best guess with garbage bags... Needs to be a bag long enough to reach from your toes to the back of your seat - or duct tape a couple bags together to make it that long. Now sitting in the bag in the kayak, bring the top (Was the side of the bag) up and over the cockpit rim and hold it there with a piece of bungee cord.

This should get you into the ball park so you can cut and sew the sock itself. Once done with that, you should be able to take the real sock's extra cloth up and over the rim, hold it in place with a bungee and then tack down the hem around the coaming. (The bag should be inside out when doing this - with the seams inside the bag.) Once tacked down, remove the bungee, sew the hem and then put the bungee back in.

Should be done by then.

You may want to sew the seams of the bag itself with a "flat felled seam" which a look through the net should find several instruction pages to show you how to make one. This is a particularly strong and can be a waterproof seam.

Let me be clear... I have not done this, but this is the approach I would take since I do not have a pattern. I expect it has a better than average chance of success in the first try, and should use less than $10 of material if you need to do it again to correct any problem found.

Rick