yeah i'd considered that and had thought of other ecentric type adjustment as it is flange mounted, the trouble id that the end of the shaft needs to sit in a pillow block bearing, so i thought i could perhaps mount both the bearing and motor on the channel and slide up and down to tension.

I've been searching all night and other than making a one off bit of channel it looks like too much effort, i've though about making a spring loaded tensioner instead, not quite what i'd like but beggers cant be choosers