Not being satisfied with the sturdyness of other belt clamp options, here's another.
(Main photo doesn't include the clamp block, but it is in the scad/stl.)
This part assumes that your belt is a 1/4" MXL, and that the teeth face up. Adjust as desired in the source. The scad renders two parts -- called Upright and Top -- which you clamp together to hold the belt. Holes are intentionally tight, but there are constants to let you open them up if you'd like. I've minimally tested it, please send/publish improvements.
2x M5x15 (or longer) cap screw (McM 91290A231)
2x M3x20 (or longer) cap screw (McM 91292A123)
2x M3 nut (McM 90591A121)
2x M3 washer (optional, McM 93475A210)
- Print enough (see below)
- Insert trap M3 nuts with part upside-down, making sure they're flat
- Fasten part to extrusion end (keeping M5 screws loose), with top tight against plate, tipping inward
- Sandwich belt hand-tight, add M3x20 screws gently so the nuts don't fall out
- Tighten M5, belt should twang. If not, loosen, move by a tooth, try again
- Verify left-to-right alignment of belt, tighten M3
Print 4 copies for a single-y-belt design, or 6 copies if using double-y-belt.