I used a straight edge and found a couple of places where the edge was slightly proud (rather like a fret rocker), then used a couple of long pieces of wood with the grille sandwiched between them as suggested by @sandy_r to hold it in the workmate with the wonky edge uppermost (as it was landscape rather than portrait it didn't fit between the threads of the workmate). I'd used masking tape to put along the edge so I could see the sticky-out bits, and attacking them with a power file sorted that out. Once I'd worked out the approach, this was the best solution as it was the grille that was misshapen, not the cab.
So then I came to putting the LF driver in, and one allen bolt crossthreaded and jammed firmly and I managed to push the T-nut out of the baffle trying to extract it. I can see enough of the shaft to Dremel it off when I regain my morale. I've cut a groove in the baffle where the T-nut prongs go though so I'll need to make that good - araldite? I'm going to put a board over the driver while I perform the operation to try and avoid putting my hand or a power tool through it. It's just one thing after another, with the finishing post in sight.