I made my own little belt shortener for my Ultimaker. Some of the ones i found break apart, others are very big, or need a ton of Skrews. Some of my Beltshortener started to wrap a bit after like 100h of printing. So i made a v2 which is a bit stronger. ShortenerV2.stl Print it, slide 2 of them over the Belt, use a Screw to Thighten them. PS: You may need to Drill the hole a bit, if you want to use 3mm Skrews.