This is an anet a8 frame anti bending support.
unlike all the othet solution this require you to buy something form you local store:
1 M8 threaded rod
3 m8 nuts
3 m8 autolocking nuts
(i spent 1,70 euros at my local store for all the pieces)
there are 4 items to be printed
front and rear support for the front side of the frame
front and rear support for the rear side of the frame (the are 2 files only one should be printed depending of you frame version)
cut the threaded rod into 3 pieces at least 3mm less than the distance between anet frame supports.
use autolocking nut on one side and the other three on the other side
install them from the bottom of the printer
all support fits in already there screws hole, the rear-rear fits in the y axis motot support.
i used anet spare screws and nuts to fix the parts to the frame
see installation pictures