Yup. Another tensioner. Except this one's really simple. And small. And bullet-proof. This one simple piece works for both the X and Y belts on the Jr. and probably any other printer that uses .2" pitch 1/4" width timing belts.
Grab a bag of #4-40 x 1" machine screws/nuts from Home depot. Press-fit the nuts and thread the screws through the tensioner and captive nuts. Pull belt taught and press the tensioner on. Fit will be snug. Fine-tune the tension via the screws. Play Deliverance on your now banjo-tight Printrbot belts.