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.
Added 6mm versions for smaller threaded rod support of 5-6mm.
Print at 0.3mm with 30% infill.
The top bracket mounting holes line up with the existing holes for the z-axis linear rod support. You will need longer M3 screws to accommodate the extra thickness the bolt will have to travel through - 25mm length is good.
For the bottom bracket, you will need to install a longer length of 10mm threaded rod by an extra 200mm. This will allow 100mm extra to extended on either side of the frame that the bottom bracket can fix onto.
Finally you will need 8-10mm threaded rod with length of 350mm to connect the top and bottom brackets together. Tighten with nuts on either side of the brackets.