A simple little belt tensioner. This uses a zip tie for tensioning, and has no sharp edges that the belt can abrade against. Just print your preferred size (file name says how much the tensioner can tighten each belt. Slip the main part of the tensioner over the belt, and feed a zip tie through the hole, around the belt and through the small part. Then just tighten it up.

I developed this as a simple way to equally tighten the three belts on my Kossel, but it's applicable to all printers that use those little spring tensioners.

It does need supports, but the supported area doesn't have to look nice.