When I installed properprinting's ender 3 belt driven dual z there was some slack in the belt so I created a small belt tensioner for it.
Ender 3 dual z: https://www.thingiverse.com/thing:3746653
Parts needed:
1x ball bearing, 16mm diameter, 5mm thick
1x m3 ~9mm long
2x T nut
2x 2 bolts for T nuts
The author hasn't provided the model origin yet.