Simple spruce 2 by material (studs basically, from Lowes/Home Depot). Don't use pressure treated.
Stainless steel screws are ideal for attaching it. Sheet rock screws are not, and they are a favorite for previous owners of boats I buy, for whatever reason. Some guys like to use Simpson strong ties to mount the wood to the ribs, but the problem with that is that they are usually galvanized, and galvanization touching aluminum is not a good idea. I would get aluminum channel that whatever size wood you use fits in well, rivet or screw it to your ribs, then put the wood in the pocket, and through bolt or screw into the wood.
Go to the completed modification thread, or whatever it is called that Jim has stickied to the top of this forum and view the many projects. Best links there are to get the ideas you are looking for.