Need help/advice about putting a wing cover on for a Pterodactyl Ascender

My friend and I are working on repairing an old Pterodactyl Ascender, and are trying to get a wing cover onto the frame. The cover is new... or at least it was once, about 15 years ago. It's never been installed, and we're having a devil of a time. The material seems in good shape, but it is un-yielding.

Try as we may, it appears to be just a bit too small.

So here are the questions...

1. Are there any general tricks to the craft that we should know?
2. Are there ways to soften the material, allowing it to perhaps stretch a bit more?
3. If the above fail, where do we go to get a new sail cover? What vendors can we reach out to?

We believe the original vendor was something like "Great Lake Sails", but I can't seem to find them on the net. As I mention, this was quite a number of years ago, and we've lost the records.

Thanks to all.