This X-axis tensioner is a remix of http://www.thingiverse.com/thing:1683070 which is a nicely designed tensioner. For the AM8 frame application, that the frame was narrower by about 25mm. Locating the idler inside of the right X-axis carriage causes some bed area to be lost.
Additionally, in my experience the small idlers with a bore fitting an M3 screw do not last very long. The small bearing can not take the load of a tight belt for more than 100 hours of operation.
This tensioner addresses both issues. For this tensioner you will need the larger idler with a bore accepting an M4 screw. You will need a longer belt. The nuts should be “melted” into the body of the tensioners. For this you will need a 50mm screw to use as a tool.

Anet

A8

No

No

200

50