And if you think you might be experimenting with different contrast parameters, you might get a set of filters for a decent price. They'll usually come in a leather case, with room to hold the adapter and a hood. Watch out for the old Wratten filters though, they usually haven't aged very well.
I just got a 30mm adapter for my Argus 40, and am looking to use the hood from a Bay 1/Series V adapter (for a TLR), as most Series V hoods are quite large.
PF