Canadian Greenland paddling coach, Paulo Oullett used to custom-sew really nice Goretex tuilk's but stopped a few years ago (either he didn't have time for that or material shortages in the wake of the covid slowdowns made it too difficult). He even had designed pants that would mate with an inner sleeve that he could add to the tuilik for added cost. I wish I had not delayed ordering one until it was too late. I did find an old article by a paddler who reviewed Paulos's tuilik and describes it in detail which might offer you some tips on your construction:
https://qajaqrolls.com/custom-tuilik-by-comfort-paddling/
I have considered writing to Paulo and suggesting that he sells a pattern and instructions so people could make their own from his designs. I've sewn lots of outdoor sports gear over the decades and would love to try that. I did buy a pattern for a neoprene tuilik years ago through this site and still have not tried it. For a non-neoprene one, be sure you make the tube connection between the deck and the body of the garment extra long so there is enough slack to allow you to lay completely forward and backward onto your boat deck plus be able to slide off to the side over the gunwales during balance bracins or rolling maneuvers without having the tuiliq pop off the cockpit coaming. It won't matter if there is some loose material around your waist because of this since it will just sit in folds.
Seattle Fabrics sells outdoor clothing patterns including some for shell pants. They also have a cagoule pattern (long bodied hooded anorak) that I have bought and thought I might try using it as a basis for sewing a storm cag or even a Goretex tunic by adding a deck to the hem.