This back frame bracket is a deep redesign remix of following parts from Henkolizer and MonkeyDudes:
-https://www.thingiverse.com/thing:1912107
-https://www.thingiverse.com/thing:2284888

Changes performed are:

• Wider bracket to properly anchor the X axis. Now, bracket touches both sides of structure to make it stiffer.
• Two holes have been added to also anchor the bracket to the central cross bar to avoid any bending.
• A flat surface has been added to fill the gap between central crossbar and the platform in which the printer is placed.
• Some model errors have been fixed which caused rare slicing issues.
• Some parts have been lightened in a beam way to reduce the ammount of material and the printing time.
• Dimensions have been checked and fixed to meet the printer dimensions and to ease the assembly process.
• Four vertical reinforcements have been added to increase the stiffness. EDIT: a new version without two vertical center reinforcements has been upload to ease the assembly (the original one can be mounted perfectly, but the new one allows more space to use the wrench without any obstacle).
• Motor stop bracket shape has been changed from a rounded one to a corner in order to ease the print. EDIT: In case you prefer not to rewire cables from right side a slotted version has been uploaded.
• Bridge parts have been rounded to avoid using any support.

I printed it in PLA and it works perfectly. You obviously need to rotate the part 45º to print it.

After printing, some sanding might be needed, however, you can use the horizontal expansion in Cura (or equivalent in other slicers) to adjust according to your printer calibration. Remember that negative values of horizontal expansion add clearance and positive values increase the perimeters.

It is highly recommended to make the part to be tightened in order to increase the stiffness. For center cross bar assembly I used M3 x 20 mm screws. Obviously, you can screw the whole bracket to the platform by using M4 countersunk screws.

I appreciate any feedback. In a future version I will move the vertical reinforcements to ease the assembly, although it is very simple.

Hope you enjoy it!