I would try a local motor trimmer or canvas goods manufacturer first. We had a family business for almost 40 years down in Moorabbin, Victoria and made 100s of tonneaus. At least with a custom made one you get to decide how far down the sides you want it to hang and also how you want it attached at the front ( ropetrack, body post, press studs). Then there is weather you want pull down loop, continuous elastic rope or press studs for around the sides and makes sure there is a rub strip underneath like felt or the like to minimise scratching.

Cheers
Mike