/diy/ - Do It Yourself

>>2609645
Lots of ways you could handle this.
Within TinkerCAD, I don't know, not aware of a way.
In Fusion or other CAD packages, very easy.

Given where you're at with it, I'd make the cut with no adjustment so they're identical in size, export the STLs, then take the piece you want to modify/tolerance and print it with x/y size compensation or hole compensation.
Picrel is the setting in PrusaSlicer, there is an equivalent in any slicer worth using, some even have separate controls for expanding holes as opposed to enlarging/shrinking external perimeters.

I'd print the U with some amount of negative x/y compensation, you can adjust as necessary to get a nice fit.