I have built the Prusa i3 Bear Half (https://www.thingiverse.com/thing:2562174)
which is a Prusa i3 frame upgrade. Cutting the 20x40 to the correct length is critical to obtaining a square frame, but in actual practice such cuts are very difficult.
These shims will compensate for the minor variations in length of the 20x40 and will also soften the metal to metal joints.
these stl files were a variation of Isaac Norris nice thing (https://www.thingiverse.com/thing:1729544) - thanks Isaac!
These are very easy and quick to print - no special setting, print several of each so you will have the selection needed for the build. I printed mine with PETG which gives just a little softness which PLA does not.