While building Allted's excellent MPCNC project, I wanted to try and find a little more flexible belt mounting solution. I liked the Belt Tensioner + Corner Block by tincho85, but wanted to avoid printing new corner blocks and reduce additional hardware. I also didn't want to modify the corner blocks.
This design has two printed pieces per belt end, an anchor and clip, and uses only a 20mm M3 bolt and nut per mount to provide about 6mm of adjustment between both ends. The anchor clips into the corner block where the zip ties normally attach. This allows you to easily detach the belt by unhooking the anchor after relieving tension on the belt. The maximum amount of lost travel is probably a few millimeters at most. I designed this for the J size corner blocks, but I don't see any reason it won't work for all the sizes.
Assembly can be a little fiddly as you set the initial belt tension with the clip and slide it into the anchor. A caveat is that my MPCNC is not fully functional yet, but so far, this solution seems to be what I was looking for as far as a drop in method for mounting and tensioning the belts.
UPDATE: Added a version of the anchors for the lower belts to hopefully clear MPCNC builds where the tubing extends past the corner blocks. Thanks to cyde for spotting this and his remix. I have not tried these personally and if you need even more clearance check out the remix.
.2mm layer height, .4mm nozzle
Printed with Microcenter Inland brand black ABS at 235 with 3 layer perimeters.