David wrote:

Why use a straight edge when he said the wall is uneven? He needs to
sand to a scribed line, wouldn't you think?


But tricky to scribe to a line if it won't fit (have to hold at an
angle, I guess; less than ideal).

I'd guess that the walls are relatively straight, just not perfectly
square.

OP: if you go with a cutting approach rather than sanding, know that
the glue in the plywood will do a number on your blade, so be ready to
sharpen or toss the edge used. How about your circ saw with a clamped
straight-edge guide for the left side of the saw (assuming blade on
the right).
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