I have a few chests, both open and closed, that would probably be in the ballpark of what you\'re looking for. I got them from Iron Wind Metals (previously Ral Partha) at a con, but I can\'t seem to find them at their online store, but maybe you could email and see if they have any for sale. Hopefully this links to the Rackham ones James mentioned. I\'m not sure this one will work, but it\'s worth a look. This is probably my favourite chest ever, but I doubt it would work for the mimic concept.
i think most manufacturers have some kind of chest, reaper does...and gw has at least one. look around a bit, but if all else fails, why dont you try your hand at sculpting one?