The insert that comes with the rectangular hatch is weldable and Hobie makes a welding kit for around $60. Most elect soldering irons will also work. Get a 3-5 gallon plastic bucket to pratcise on it is simple. Cut the inert 1/2 way up, cut a ring 1/2-3/4 inch wide, put the two halves together, use ring as weld filler material. Should be strong and water tight. Can be welded from both sides.

TIP: Lightly duct tape inside, then tack in a couple places outside before removing tape and filling whole weld. Take your time, get a good fill and it will last forever.