I downloaded all of the Mirror Holders I could find here on Thingiverse for a TEVO Tornado, but none of them fit the 12" Mirror Glass that I purchased at my local home improvement store (HomeDepot - $10 for 6, 12" x 12" x 3/16").
The 12" x 12" x 3/16" Mirror Glass dimensions convert to 304.8mm x 304.8mm x 3.5mm.
All of the files I found were for 300mm x 300mm x 4mm glass.
So with a little help from Tinkercad, I resized the recesses to 305mm x 305mm (I figured that printer inaccuracy wouldn't notice the difference 304.8mm to 305mm) and I also took .5mm off the top to match the height of my mirror glass.
It worked out great !!!
With the 4-corners installed, the 12" MIrror Glass fit perfectly (see photo).
After manually leveling the bed, I ran the G29 command to check the "Autoleveling" (I have a BLTouch installed) and where previously the bed had about a 4-micron swing, it now has a .5-micron swing.
Glass, mirror glass, mirrors are so much flatter than any print surface I have ever used.
To those who might be wondering, I recently switch to a TEVO Tornado after my Printrbot Simple Metal died (controller board). Printrbot closed their doors Oct 18, 2018. I plan on using the Printrbot parts to build a HyperCube.
And, yes, one of the first things I did was remove the BuildTak surface from my Tornado.
Remixed from https://www.thingiverse.com/thing:2796469
Inland PLA or PETG
You can use either PLA or PETG for these parts, as the corners of the bed do not get hot.