This is a support for the spool holder given out for the MK3 printers. I made it a while ago and realized others might want it as well. I noticed that the holder would bounce quite a bit during prints, which worried me. It may be completely unnecessary, but I like it. You can just slide either side back and forth to change the spool. At first, it may be a little stiff but will slide more easily with use.

Here is the spool holder it fits:

https://www.prusaprinters.org/prints/88353-spool-holder