slack roof panel fabric

[lewdon]

I have just re-coloured my roof (not yet waterproofed) but the fabric appears to have stretched on just one of my roof panels, causing it to ripple slightly, which doesn’t look good when the light shines across it.
I am reluctant to try and peel back the vinyl lining to get at the glued edge of the fabric so that I can try to re-tighten it, particularly as we are now coming up to the time when I want to drive the car not work on it. .
Has anyone any ideas how I can re-shrink the fabric (I have already tried wetting it and then drying it with a hairdryer) I was even wondering whether boiling water or a shrinking dope might do it.
Any and all ideas will be very gratefully received..