Hate to say this but from what I remember of the Thermarest tent ends, they mounted differently and were supported differently than conventional tent ends.
There may be more than just new canvas.
I would first contact Bear Creek, since they've been around for years.
They also might might take your old one and fix it or make one with the old one as a pattern.
Unless they already have made replacement ones.
The Thermarest tent end was unique and had some positive features.
But issues with water drainage and their infamous futon-style mattresses doomed the Thermarest and left the Kodiak/Cub hybrids with bad rep.
That brand hasn't been the same since.