This is simple remix of the string tensioner posted by stewarma.

Replace the screws in the printer bed with these and some nuts and you can adjust your X axis string tension by tightening the screw.

STL includes two parts, which are mirrors of each other; one for each end of the print bed.

This version extends the part the bolt fits through to the whole thickness of the base, so that you can use a nut on the end to hold the screw tight.

Since this design is meant to be used with a nut, the hole exists in the model for the full depth, unlike the original.

STL includes two parts, which are mirrors of each other; one for each end of the print bed.

Note

If you don't want to lose any of your print volume in the X axis, you'll need to cut some material off of your Printrbot Simple. You'll need to cut out one or two notches, depending on if you use one or two tensioners.

I've included a pic showing the notches I've made in my Simple (even though I'm only using one tensioner on the Right side of my print bed) as well as a pic of the tensioner with the printer in use.

Disclaimer

Any modification of your Printer is taken at your own risk, with no warranty or guarantee provided by myself nor Thingiverse; either express or implied.

That said, if you do, make sure you keep your bearings clean! Cover them before cutting.