I'd do the boards I think and have a card slot in the corner. And perhaps to get around people thinking you've done all the boards, you'd just need to sort out the wording somehow. Depends on how much space you've got to play with but if it's enough I'd have something along the lines of "Display boards with the following trustmark have been tiled by Andy Allen, Tel:032032030303" and then have a small brass etched square thing on it with your name or logo on too. So then they look out for those boards.
Not sure if that needs more though :lol: - Doesn't sound right reading it back. But something like that should do.
If you had a card holder in the corner and a brass card sized plaque type thing with the wording on it in the other corner, that'd stand out from the others.