I use this to check that a packaged item is thin enough to still fit into a Large Letter that has the maximum dimensions of Length 353mm Width: 250mm Thickness: 25mm Large letter, letter & large letter+letter size guides. 3mm, 10mm, 15mm thick wall and screw-hole variations included as a Fusion 360 file with parameters, STEP & DXF.

Most of my designs are thin enough to be posted in a letter or large letter, I do this to make postage as cheap as possible. Designing to these requirements results in cheap, quick and convenient prints.