I needed a solution for joining T5 timing belt and after trial and error best solution was to melt the belt together with a soldering iron at 260-270°C, but to get belt to overlap good and melt as much as possible together, a 3 finger joint was preferable.

This jig lets you place a thin knife blade (0.8mm) into a 3D printed block that fits into a belt holder (belt is 10mm wide)
and cut slits in the belt at even distance.

Then you have 2 belt ends with 2 slits in them, you need to remove the middle belt from one end, and remove the two outer belts from the other end, creating a finger joint.

Use the t5-belt-holder to place the two ends into and hold the position correct for the teeth to match, preventing pitch misallignment, then melt the fingerjoint together with the soldering iron or some better suited method.