This was going to be a belt tensioner for the larger original Ender bolts/rollers, but in between starting this and completing it I bought some toothed wheels and so it fits the 5mm screws for that instead. I may remix a version for the originals in the future, the frame should have enough volume to fit it.
When I cut the new belt for my longer bed I had quite a bit of slack so the motor and tensioner were literally right at the ends as far as they could go. To compensate I have given this tensioner a good bit of range. Because everyone knows it's easier to design and print something like this than it is to take the carriage off and trim some belt, right? :)
EDIT I've now added a shorter (40mm) frame and pulley, which are more suited to the stock Ender as it doesn't need the extra range that my Y-Bed extension did.
I used supports under the puller just to make sure there were no issues.