Thanks to all for the suggestions. Looks like the QuadBox is the best
solution available, so I'm going to run over to my local Ace in a bit and
see if they have one in stock.
In case anyone is interested, here is a shot of the location in question,
with the nut already removed. The nut in the foreground is actually 1/16"
smaller than what should be in there, so you can see that the flange is
quite close to the nut.
And the angle of that shot makes the area seem rather roomier than it is,
here's another one (other side, with the pan/flange removed) that better
shows how close the frame member is:
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