There are several rear frame braces for the Anet A8 3d printer. However all put stress on the 2 screws holding the stepper motor firewall which in my opinion isn't the best approach.
The optimal setup would be to brace the stepper motor itself but I didn't like the idea because the heat from the stepper motor would not dissipate as well. I therefore decided to do the next best thing by bracing the firewall which supports the stepper motor.
The frame brace is designed so that you only need to loosen the 2 rear nuts on the middle frame support and slide it underneath. Then re-tighten the 2 nuts.
See pictures for more details.
Make sure that your slicer program is set to 100% scale. You will also need to rotate to object to make it fit on the heat bed.
Be sure to use a front brace as well e.g. http://www.thingiverse.com/thing:1857991
Please let me know if you have a problem with the design so I can improve it if necessary.
Also please post a picture with comments if you have made one.
I've added a slot for the cables.
I've uploaded V2.0 which solves the problem of different rear frame dimensions. Users that have have modified there h-carriage mount can now also use this frame brace.
I've uploaded a lengthened version (+25mm) "Anet_A8_Frame_Brace_Rear_V2.0-25_Leo_N.stl" which was designed for this project: http://www.thingiverse.com/thing:2251940
(02.08.2017: File has been removed and is now available on the project page.)
Select a minimum of 0.8mm shell thickness and top / bottom thick layers.
I recommend 1.2mm.