I'm going with exposed plywood on my 4x4x8 smoker because I've decided to never go above the mid 200's. However, I'm going to run it hot and empty with heavy smoke for a few hours before actually using it to allow any nasty fumes to off-gas from the plywood. I'll also treat with mineral oil or food grade linseed oil as well to help season and seal the wood from moisture. Might not be a bad idea to give the plywood of yours a wipe-down with some before you cap it with metal, prolly make your smoker last longer (should be plenty durable without it, I just tend to overbuild, lol). I understand if some folks still worry about fumes and prefer to line with some kind of metal. What kind of caulking are you thinking? I'd go with 100% silicone as a safe bet. Also, depending on what kind of smoking you're going to do, I'd use untreated sheet metal as a last resort due to the potential for rust. Had a little store bought smoker years ago that started to rust somewhere up in the vent, condensation from what I was smoking caused the rusty water to drip on my food, ruined a few batches before I figured out what was going on. Back in my hometown in Alaska pretty much every smokehouse is made of old ratty galvanized corrugated roofing, but they're big and only used for cold smoking fish. Some guys even shoot their smokers with a shotgun after it's built for ventilation if the metal is too solid
Best of luck with your smoker, don't forget to post some pics of the finished product!