Looking for an Invacare Max Contour air cushion

[heartdog]

Thought I'd put this out there....I've gone through SO many cushions in the past couple years. It's like the Holy Grail trying to find the right one. I have an old Invacare Max Contour air, that I used as my car cushion for a few years, until deciding to try it on my chair again. I've always assumed it was almost exactly like a Roho, with those air pillows, but for some reason, it feels different.
So my problem is, the air insert cushion part has suddenly gone south, and the seams along the edges are coming apart, letting air out. I've tried glue, and now am using "insta-grip" rubber goop stuff to try and fix it, for now. But I'd really love to get a hold of a new insert.
I'm always on the verge of my inner ischial tunnels breaking down, and this pad is the most comfortable and doesn't seem to cause irritation. I can't afford to buy a new one at the moment, and it's super hard to find a 15X15, especially to demo. I don't think the air part would have to be specifically for that size, just not too much bigger or smaller. I imagine a 16X16 would work, too.

Thanks!