Yes there are a few 3D printed spectrograph designs floating around, (though none for a high resolution Littrow that I can recall) The current one generating interest on the astronomical_spectroscopy yahoo group is Paul Gerlach’s LOWRES
https://groups.yahoo.com/neo/groups/astronomical_spectroscopy/conversations/messages/13914
https://www.thingiverse.com/thing:2455390
Christian Buil has also been dabbling with 3D printing for his experimental UVEX spectrograph
http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=1773
Robin