thanks everybody for the help and support. the neg holder seems to work fine. keeps the neg in place and it's resonably tight.
the only thing that bothers me it's the fact that it's not solid when slided in the head. actually I can tilt it with a little pressure.
if you're interested about the head alignment, here's a picture I took. as a reference, there's a hotshoe spirit level.

@phenix: I can fit just 50mm lens and I am pretty sure that the lens board (that threaded 'dish' where the lens screws into, if I understood correctly) is mounted properly..