Sorry to hear you are having problems with your Lhires III. Do you mean light loss from the spectrograph, or light leakage into the spectrograph from outside?
Will you be coming to the Observers’ Workshop – Variable Stars, Photometry and Spectroscopy on 29th September? That is a good place to start as there may be other Lhires users present. I can try to find out if you are interested. I would not rule out an Lhires III weekend, but we would need enough interest to make it work, and people willing to organise it. Also a weekend event has much bigger costs due to the need for overnight accommodation.