This model is designed by ltlowe (http://bit.ly/2FZxDip), thank you very much for your great support. For details, please see the description below:
I've been looking for a Y-Belt tensioner for the AnyCubic i3 Mega for a long time so I finally designed my own.
The base mounts with two of the bolts for the Y-rods to avoid having to drill holes in the case to mount the base assembly. (the original M3x8 might work, but I used longer M3x12)
The idler pulley should be mountable with original screw (M5x16) and nut but it is very close. M5x18 works better. M5x20 is just a little too long and scrapes the case.
The tension bolt should be an M5 hex head cap screw, with the head going inside the carriage and the nut in the knob. I used an M5x35, but it's a little too long. M5x30 would be better.
The pulley carrier originated from http://bit.ly/2tUcd1k - "Anet E10: X belt tensioner by MrCrankyface". I have added the hole for the nut and enlarged the hole for the tensioner bolt to fit M5 instead of M4. When inserting the pulley, it is very tight, but as long as you don't tighten the bolt too much it will loosen up.
V1 - 20180210
The bolt hole doesn't go all the way through the front pate, only half way.
Printing the knob in the default orientation without supports caused some in-the-air printing for layers 38 and 39 as it transitions to the shaft.
V2 - 20180217
Fixed the bolt hole in the base.
Modified the bottom of the nut hole in the knob to make printing easier without supports
Chamfered the outer edge of the nut hole in the knob to make it easier to fit the nut.
V3 - 20180313
Replaced original 123d file with correct one to match v2 stl files.