I do agree they ought to keep the booth clean, as you said the stuff stored up there is worth a lot. However, I don't think you're going to be able to force them to clean the booth, doing so might come accross as bossy and nagging (kind of like when their mothers tell them to go clean their rooms
). Asking them every once in a while to do a bit of upkeep and mentioning your reasons for concern, as well as offering to pitch in if you have the time, should help. If it gets to be a real problem though I would take it higher up.
