Cool. Is it a Grumman?
There's a local Burning Man truck that has an aluminum body. The finish is swirl marks from an angle grinder. I'd go with 6-8" overlapping circles.
About the Faraday cage, there may be RF field strength meters for testing the cage-iness as close as your local Radio Shack. Oh, wait.
It has one sliding and one roll-up door? It may need [ionically compatible] grounding straps or electrically-conductive grease on the rubber seal?