This design incorporate an additional spring on the free end of the screw, that makes tensioning more secure. Adjust with the screw until the belt "twangs" as desired.

The other interesting feature (compared to the previous design), is the slightly thinner plates: if you omit the washers, the motor mounting screw reach the bottom of the stepper holes before they fix it completely onto the plywood. For this,, you'll need another pair of 25 mm screws + one nut and one washer for the top holes, so the motor is let loose just enough by its 4 screws so it can slide vertically.

See it live: youtube.com/watch?v=ow2jrRFdzb8

I hope that any belt length variation during a long print can be compensated by the spring. It may happen when it gets hots, or when if wears out, though I am a bit skeptical.

Now, the parent thing is simpler but also riskier for some people as it makes it easier to break something if you tighten the screw like a fool, check it here (with how to mount): thingiverse.com/thing:34785

BOM:

- a 3mm drill

Fixed plate:
- one M3 nylock nut
- two 10mm M3 screws (to attach the plate that holds the nylock)
- two regular M3 nuts for the previous screws

Tensioning screw:
- one long 30mm M3 screw (to tune the tension and hold the spring)
- an M3 short but robust spring (those of the UM bed are almost too long but still ~OK: for this reason I made one part ~2mm longer -- to reduce the gap between the two parts--), but the holes are still in the same place so upgrade is possible without having to drill more holes. I used a shorter and stronger stainless steel spring that is about 10mm long and 4.8mm OD and 3mm ID (ie. it fits better the 30mm M3 screw). With this spring, I only need a few mm of compression to achieve a correct tension, sorry but no exact reference.
- two M3 washers on each side of the spring (one is hidden within the top part hole, to prevent the spring from damaging the plastic in the long term -- it is probably overzealous, and you could "steal" some more length for a longer spring than mine if you omit the washers)

Sliding/motor plate:
- four 25mm M3 screws to replace the stepper 20mm stock screws
- two M3 simple nuts + two M3 washers for the top two screws

The latter two items are not compulsory, but they let the stepper motor gently slide freely along the plywood wall. As for me and my steppers, there were almost no freeplay left when I screw them at the maximum in the stepper motor, but still, the spring I use is strong enough to pull it downwards nicely back when I move the stepper. Really OK.

Note: I mounted one of these tensioners on my Ultimaker because I am curious to see how/if printing could provoke unwanted oscillations when the motor is kept lose. If it does, I will secure it by adding washers to the stepper mounting screws, once the belt tension is set up (or I'll even revert to the inital spring-less design).

Update: one month later, I cannot find any negative impact on the quality of my prints (see the remarks below btw). Also, I will add kapton tape between the moving part and the plywood wall to make it slide a bit better may be.

Final note for the curious: the dirty plug on the picture simply lets me unplug the filament feeder, more on betterprinter.blogspot.fr/2012/10/removable-filament-feeder-add-blug-to.html