Update 4/22/19 - mk3s support - It was brought to my attention that the new belt holder/tensioner used on the mk3s is not compatible with this belt tensioner. The easiest and in my opinion the best solution to using my belt tensioner on the mk3s would be to print the original belt holder used on the mk3 since it is a direct bolt in. Additionally having my tensioner with the mk3s holder/tensioner is redundant and a potential source of belt loosening over time. I have uploaded the most current mk3 belt holder so you won't need to go searching for it.

Update 3/10/18 - I used jzkmath model of the assembled prusa i3 mk3 with the r2 components and found that the tensioner still works with the r2 belt holder, maintaining a belt path that is still parallel to the bed.

Update 2/28/18 - Please refer to the picture of the two parts in Slic3r that I have uploaded. This is the proper orientation for printing the parts so that there is no need for supports.

After a lot of design reviews and changes I've finally landed on a design that I think you will all like. It is simple and works like a charm. It is available to those using the stock idler (although I'd strongly advise an upgrade to a dual bearing unit), a gt2-16t idler (no more than 8.4mm wide) or a gt2-20t idler(no more than 8.4mm wide). When downloading the STL files, please be sure to select the proper file. They are all named according to the idler you wish to use. In addition to that, the 3D parts are embossed with "s" for stock idler, "16" for 16 tooth idler and of course "20" for 20 tooth idler. The Y axis adjuster arm is standard across the three choices.
This only requires some additional hardware which is listed below but requires no modifications to the printer itself. It can be used in conjunction with any motor mount solution that retains the stock motor mounting position and it must be used with any belt holder that retains the stock belt location dimensions (stock belt holder is more than sufficient).
When inspecting any misalignment issues on the 3D assembly of the Prusa i3 mk3 created by jzkmath, I found that there was .5mm misalignment between the motor pulley and the belt holder. I strongly believe that this very minor misalignment does not warrant printing a new belt holder or motor mount. Going forward with my design I wanted to be able to mate up with the stock belt holder while maintaing great alignment and parallelism between the belt and the carriage. This tensioner maintains perfect belt alignment throughout the entire adjustment travel since the pulley never moves vertically.
This has been tested on a Prusa i3 mk3 (thanks jweaver!) and fits beautifully with no interference issues, perfect belt parallelism (by eye) and gives the end user the ability to finely tune the tension on the belt. BINGO.

Hardware needed:
-M3x35mm socket head cap screws (2 pieces)
-M3x30mm socket head cap screws (1 piece)
-M3 Nylock nut (3 pieces)
-your choice of idler pulley

assembly instructions:

1. Place the Y axis adjuster arm over and onto the idler frame mount leaving the slot for the Nylock nut facing out. Using one M3x35mm socket head cap screw and one Nylock nut to secure the adjuster arm onto the idler frame mount at the pivot point. DO NOT overtighten this as the adjuster needs to be able to pivot.
2. Using your pulley of choice, place it into the idler frame mount mounting ears and align the idler bearing, mounting ear and slot on the side of the adjuster arm. Using one M3x35mm socket head cap screw, insert it all the way through the three items and use one M3 nylock nut. Again, DO NOT overtighten this as the adjuster needs to be able to pivot.
3. Insert one M3 Nylock nut into the adjuster arm slot and using one M3x30mm socket head cap screw, insert through the ear on the idler frame mount and into the adjuster arm catching the Nylock nut.
4. Install two Nylock nuts into the provided hex holes. At this point you are reusing the Nylock nuts that came with the printer.
5. Install assembled belt tensioner with the pivot point towards the top. During belt tensioning it is important to refer to the manual as to the specific tension required. A simple adjustment of the M3x30mm screw will be all that is needed for tensioning.
   please refer to pictures for assembled tensioner.

Enjoy!