Do you mean repurpose an old one or clean up one you are using?
Cheaper to buy one. Check your dollar store. If this is for stuff other then dishes or food stuffs check your local salvation army.
For the price of all the requisite tools and materials to try to make a rack that is strong, durable, and waterproof, it may be more cost effective to purchase one.
Check out some of the expandable and collapsible styles that are available:
I found this one on Hometalk using wooden bed boards. Does that work?