I've just uploaded the newest version of the LOWSPEC to Thingiverse:
This version has an improved slit holder which will ensure that the whole slit length can be used. It's now possible to change different gratings easily and quickly via a redesigned grating holder system that uses magnets:
Unfortunately this also meant that the main body had to be changed and parts of the focussing unit.
The good news is that you can reuse all optical components, the slit holder assembly (although I would recommend to replace the slit holder disc with the new version), the guide pick-off mirror, the guide lens holder, the main mirror holder and, the collimator holder.