Mordheim Accessory sprue has a nice one on it.
I've got a bunch of those knocking around from ages ago, guess I should assemble them.
Mage Knight dungeons also had nice-looking treasure chests in each box, so you might be able to ebay some of those on the cheap.
Edit: Yep, I was right:
http://www.ebay.co.uk/itm/Lot-16-Mage-Knight-Dungeons-Chests-MK-Dun-1-/190634312176?pt=LH_DefaultDomain_0&hash=item2c62b0c1f0Remove the dials from the side, fill in the top with putty and do a little repaint and you're good to go.
They're a bit chunky, and might be slightly larger lengthwise than a DKH square but should do the job. I used to have a load of those, and the sculpting is quite nice.