Make sure you waterproof them or use a waterproof material such as HDPE (commonly used as cutting board) for automobile door use. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
I don't, but don't go with MDF. Doors are wet environments and MDF does not stand up to water. You should get aluminum or solid plastic (e.g. HDPE) rings. Exterior grade plywood will most likely also be okay. If you get MDF or interior grade plywood, I recommend coating them with spray-on rubberized undercoating from your local auto store.