Update: before you cut you belts, you probably better try my banana XY blocks! They replace the ugly wooden block that try to both hold the head rod and to tighten belt (and they fail at both in my opinion).
So the best I found definitely were dumb zip tie tensioners !
Just cut your belts and use as many ties as needed. One of the best thing is that you can tighten the belts further very easily with the help of a small set of pliers.
In the picture on the right, you will see I also need to address the X/Y carriage block plywood cracks before it is too late...