We are also a disgruntled Heki user...
Every time out I keep waiting for the Heki to fail.
Ours developed a small leak around the combing which we were lucky to catch very early.
It was quite a simple job to take the thing apart and remove the whole hatch assembly leaving just the hole cut into the roof by the RV company.
Once down to the hole only, in the roof, all you would have to do is get your replacement Marine quality hatch, mount it on a piece plywood, then take the plywood with hatch installed, screw and seal it over the Heki hole. Use lots of Dicor and your good to go.
You could use any hatch style of your choice that was smaller than the Heki which is too large anyway.
I would not worry about trying to find the exact size replacement hatch.
