I use the Laser Engravers at the TechShop San Jose, http://www.techshop.ws, to engrave the bottom of turned wood bowls. Since these bowls are often turned from green wood, they tend to warp as they dry. This presents a problem in that the bowl bottom is not parallel to the laser engraver bed when the bowl is inverted. This would result in an out of focus situation.
The simple solution to this problem is to use a couple of bags of dried beans to prop up the bowl so the bottom is level.