It might well be that they did not have a monument. The lair lists looks like the best bet. 
These I believe are held in the relevant District offices throughout the Dumfries and Galloway Regional Council area.
It would be worth exploring 
http://www.dumgal.gov.uk/dumgal/for a contact. I phoned them a few years ago and they were very helpful and passed me on to the appropriate office. Unfortunately the lair books were not extant for the time I wanted but the staff then referred me to a lovely man who gave me so much information, I just forgot about lairs  

Sessions minutes might also have information but  they are held in Edinburgh now unless they were photocopied locally.
It might also be worthwhile contacting the D&G FHS 
http://www.dgfhs.org.uk/the volunteers there are super. I am a member so can have a certain number of lookups.
Gadget