Looking carefully at my one reference for this vehicle, I realised I had made a mistake. The left side needs to overhang the hull side, not be flush with it. No way would I want to take everything off and begin again — far easier to just build a bit of an extension and blend it in with some putty and filing after.
I started by sawing a length of the tube I used for the corners in half, lengthwise. This wasn’t all that easy, but I managed by clamping it in a modeller’s vice (the open ends of the tube against the jaws, not the sides of the tube) and sawing carefully through one side. Then I put it in with the other side up and repeated, ending up with two almost equal halves.
Those halves, I glued to 0.25 mm plastic card so I could stick the half-tubes on the corners, and then added 2 mm square rod at the top and bottom:
All that then remained was to cut a new side plate and stick it to the outside: