Spring-loaded Ultimaker short belt tensioner
by JelleBoomstra, published
Mooncactus design only had two holes for the stepper motor and two screws on the lug side, over- constraining it.
This design is intended to be mounted on the inside of your UM, between the casing and the motors. If you have and older type UM with the stack of plywood as spacers: you only need to remove one plate. If you have the newer plastic spacers you probably need screws a little longer (2mm) or you can get by with 2mm less thread in the motors?