I have been pondering upgrading my 10" i3v to M8 lead screw for awhile & liked the idea of not having to change my Z-Motor mounts. I decided to take cperiod's idea:
https://www.thingiverse.com/thing:2917730 and change it a little and round it more. I also made it 34.4mm Diameter instead of the 33.0mm he had. I loose about 7mm in the X build space versus the original M5 rods. If you want to change it to 33.0 or smaller, just change the variable Mnt_OD in the openscad file. If I need more X distance, that is what I will do. Should be easy to change this plate later on by loosening the M8xM5 coupler & taking the screws out of the plastic nut plate, pull the M8 lead screws out, replace, then reassemble.
While I was changing this part, I printed newer versions of the:
X-motor plate (https://www.thingiverse.com/thing:853811),
X-idler plate (https://www.thingiverse.com/thing:878023) &
X-axis limit switch Holder (https://www.thingiverse.com/thing:1959535)
I also printed a plastic M8xM5 coupler for the z-motors which grab 19mm of each shaft.
I have not noticed any difference in prints, but it does feel more solid now. I printed all the parts with esun PLA+ using the gyroid infill of the Slic3r Prusa edition
i3v_lead_screw_nut_mount_Mod_Rev18.stl .2mm layer height, 30% gyroid infill
X-Axis_LimitSwitchHolder_Rev12.stl .2mm layer height 20% gyroid infill
i3v_X-MotorMount_rev32.stl .2mm layer height 50% gyroid infill
i3v_X-IdlerMount_3Wheel_rev14.stl .2mm layer height 50% gyroid infill
There are openscad files for this nut plate & coupler if you want to adjust any of the variables. It is commented fairly well.