OpenSCAD Tutorial

Preview (F5) is fast and meant for iterating. Render (F6) computes the final solid geometry, which is what gets exported to STL. The special variable $preview is true while previewing, so $fn = $preview ? 24 : 72; keeps previews quick and exports smooth.

Comments turn top-level variables into Customizer controls in the OpenSCAD Playground and desktop OpenSCAD: // [30:5:120] makes a slider from 30 to 120 in steps of 5, /* [Plate] */ starts a group, and variables after /* [Hidden] */ do not appear. An assert stops the model with a clear message if someone picks values that cannot be printed.

Your task: cut a mounting hole near each corner, inset from both edges. Use one loop over two lists: for (x = [inset, width - inset], y = [inset, depth - inset]).