Having a wooden frame makes it hard to get the z-axis straight and rigid. This is an attempt to solve this. There is several similar versions already but I wanted mine to be easily adjustable so I gave it a go. It only requires putting longer threaded rod on the y axis and adding a plastic part on the corners of the frame. No additionnal holes required in the frame as it uses those of the top z-axis part.

The part is parametric so you can easily adjust it to your needs, for instance I made my y_attach part longer because my threaded rod was too short.

This design works well for me but I don't recommend it if you have M8 rods on your y axis (as it is the case with official Prusa i3) or if you do you probably need to extend both rods and make them share the load by modeling another part.