After trying several belt tensioners I decided I liked the Improved Y-belt Tensioner by Freemark the best, but there were a few things I thought could be better. The placement of the bearing was better than the tensioner by CH35C0 but I really liked the thumbscrew on the design by CH35C0.
The thumbscrew not only makes it easier to adjust tension, it also means the acrylic is less likely to be marred like the metal screws used in the original design by freemark.
Additionally, this splits the parts into 3 separate files, so if you want to print the parts in different colors, like I did, you don't have to edit the file or print parts that aren't going to be used.
I recommend a wide brim for the main tensioner part unless you know you have excellent bed adhesion. The posts are printed initially as towers and get relatively tall before they join with the top plate. This makes it into a long lever, which was knocked over twice when I was attempting a print before I used a 10mm brim.
I also recommend a wider top, bottom, and sides to give some added support. I went with .2mm all around.
I also want to note that supports are not required and using them, especially with the screw, will probably make it impossible to use.
I'm using the original design for the tensioner from Freemark. I found this was a very tight fit. I personally think this is a good thing but I had to do some sanding to get a clean fit.
Assembly should be pretty self explanatory. The posts fit into the top most square holes where the original bearing mount goes. The top plate goes over the top of the bracket. The plate slides in with the small end facing away from the printer and the screw goes in the screw hole. No additional parts should be required.
This is my very first "design." It's not really fair to say I designed it as I took 2 excellent designs and put them together in TinkerCAD. I am grateful to the original designers and the people who made a free, online tool that resulted in a very easy job making the tensioner I wanted from the best of both designs.