The stock Z top drives on my Taz 5 had a lot of flex to them and one side was cracked where the bearing gets pushed in, other designs I found on thingiverse didn't solve the flexing as well as I would've liked so I modified the stock files. Added extra material going across as well as big chamfers on the inside and around the bearing holders.
Update: Installed and works great! It seems to add some back and forth stability to the whole frame unless it's just my imagination
Recommend printing with linear advance enabled for sharper corners (I use a K factor of 0.14 for ABS on my Taz 5)
If my design helped you out tips are much appreciated! Went through a few iterations on this which cost me a good amount of filament. Thanks!