Boxes, lids and project cases from a plain-English description, with every stated dimension measured — on each part and on the assembly — before you print.
A small box with a snap-on lid, 80mm by 50mm by 30mm outside, 2mm walls, for storing SD cards.
An enclosure for a 65 x 56mm board, 25mm internal height, 2.5mm walls, with a 12 x 6mm cut-out for a USB-C socket on one short side.
A project box 120 x 80 x 40mm outside with a screw-down lid, four M3 bosses in the corners 8mm in from each edge.
A box is the easiest kind of part to describe loosely and the easiest to get wrong for exactly that reason. "An 80 by 50 by 30 box with 2 mm walls" sounds complete. It is not: it does not say whether 80 is the outside or the cavity, it does not say how the lid is held on, and it does not say how much room the lid is allowed to take out of the 30.
The number you care about is usually the one you did not say. If the box has to fit on a shelf, the outside is the specification and the cavity is whatever is left after the walls. If something has to fit in it, the cavity is the specification and the outside grows by two wall thicknesses in each direction. Say which, in the prompt, and the other one follows.
"With a lid" describes nothing. A lid is held on somehow, and the how is the part of the design that decides whether it works: a lip rim with a snap bump, a rebate it drops into, a sliding dovetail, four screws into bosses. Say which, say how tall the lid is, and say whether that height is inside the overall height or on top of it. The example below asked for a 5 mm cap within a 30 mm total, so the base is 25 mm — a sentence that resolves a number rather than leaving it to be guessed at.
Two surfaces that are nominally the same size do not slide over each other — they interfere. A snap lid needs a stated gap between the lip and the skirt that catches it: a few tenths of a millimetre per side, more if you print fast or your printer runs wide. The example below asked for 0.30 mm per side. It is also the row the first build got wrong, which is the honest reason to put the number in the prompt rather than hope for it.
A base and a lid are generated as two parts against one contract that records what each one should measure and how they meet. Each part is measured on its own, the assembled pair is measured again, and the two solids are booleaned against each other to check they do not occupy the same space. That is why an enclosure's report has more rows than a single-solid part's — and why a fit problem shows up as a failed row rather than as a surprise on the print bed.
Every stated dimension, on each part and on the assembly: the outside box, the part heights, the interface clearance, and whether the parts interfere. The report says what was asked for, what was measured, and whether the two agree. It does not say the box is watertight in the plumbing sense, that the snap will survive a hundred openings, or that your print settings will hold the clearance — those are questions about material and process, not about geometry.