The Prusa i3 frame isn't supported at the top, therefore it may oscillate or vibrate during a print.
So here is a set of brackets that will support the top of the Prusa i3 frame to the Y-Axis threaded rods.
https://www.youtube.com/watch?v=wlJzWys8nuY

Added 6mm versions for smaller threaded rod support of 5-6mm.