Here's how I do it:
1. Confirm the cabinet is level and plumb.
2. You've already confirmed the wall and floor are not level or plumb, but are they flat, or contoured?
3. If flat you can use exact measurements to size the panel. Measure from the cabinet face to the wall at the top, center, and bottom. Same procedure for the floor, measure from the top to the floor. Transfer measurements and cut.
4. If the floor or wall is wavy, find the longest measurement and cut to this. Then trace a scribe to get the contour. I use a small block plane to remove the material. If it's a heavy scribe, I'll use a grinder with 50 or 80 grit paper. Belt sander works, too.
I use a track saw to cut the panels with a 30* bevel. It makes fine tuning much easier, since I'm just shaving off the point rather than the full 3/4" panel thickness. Sometimes, if the wall drywall, and flat, I'll cut the panel just a hair (1/32") big, 30* bevel, then push the sharp point into the wall while installing. Just enough to slightly indent the paper, but not cut it. Gives a nice sharp, clean scribe.
Last edited by danrush; 10-20-2019 at 08:11 AM.