On mine the door shelves and the gasket were held in place by screws under the gasket. See pictures below. I took it apart and replaced the shelves with white dry erase marker board I got at the big box store. In my case I also extended the depth with a wood collar. A piece of drywall edge metal fastened to the wood collar allows the magnetic door gasket to continue to be useful
I lifted up the seal on mine, removed the screws, removed the shelves on the door, used a utility knife to cut the flat part of the plastic where the screws go keeping it all in one piece, reinserted the thin plastic rectangle into the seal and reinstalled the seal. Easy, peasy.