Chain guard for a belt-drive bike?
There are chainguards that work with derailleur bikes. Your belt might be wider than a chain, but it isn't wider than a triple :-)
Something like http://www.sjscycles.co.uk/hebie-chainguard-for-triple-ring-systems-with-a-48t-outer-ring-prod19993/
(If you've got a 52t chainring, it might be trickier to find something that will fit.)