The frame of my Prusa i3 Hephestos was a little shaky, so I wanted to find a way to reinforce it while changing as few parts as possible.
With only four printed parts, two 8mmx350mm steel or aluminum rods and some glue you can considerably improve your printing quality.
Print the files y-corner-left, y-corner-right, and two times holder-top
The quality doesn't really matter
Saw an 8mm rod into two 350mm pieces
Replace the y-corner parts of your printer with the ones you just printed.
Stick the two rods into the holder-top and add a little glue
Stick the assembled rods with the holders onto the printers frame and into the corner parts. Add glue if necessary
Important: I designed this for a Prusa i3 Hephestos. So the distance between the corner part and the frame should be about 90mm for it to work.
EDIT: Added step-files for customization