Just a quick test piece.

Pieces are designed without tolerance. The smaller square is exactly the size of the hole in the bigger square. On a well calibrated printer/slicer combo, the pieces should fit together perfectly. Hard to snap in, but still fitting without bending a piece. And hard to push out again as well.

STL dimensions are rather small (10mm side length for the big square, 6mm for the small one, 1mm height). Scale in X and Y dimensions accordingly, small tests don't really tell you much :)

Hint: Cura's "horizontal expansion" is the key to compensate for material width. In Slic3r, I think it's called "XY compensation". Dunno, I don't use Slic3r