For those that do not wish to upgrade to the openbuilds Y rails project but still want a belt tensioner, here is the belt tensioner portion of the project for a stock Mini. This is modled off the Mini 2 idler end, but should fit both the Mini 2 and the Mini 1 without modification. You will need 6 M3 heat set inserts, 2 M3 12mm bolts and 2 m3 washers, 2 1/2" long 1/4" smooth nylon spacers, the stock mini idler hardware, an m4 12mm bolt, an M4 nut and an M4 washer.
Insert all the heat set inserts, use the M4 bolt to pull the nut into the idler body, then use the smooth nylon standoffs and M3 bolts to attach the main assembly, and use the M4 bolt to adjust the tension. With the system loosened as much as you can, install the stock mini idler hardware and tighten the belt, then use the M4 tensioner bolt to add tension as needed.
This version is identical in function to the Openbuilds V-slot Rail version found here: https://www.thingiverse.com/thing:3451205 , this one just uses the stock lulzbot rods instead.