Would a well grounded .30 or .50 cal ammo can be enough faraday cage to protect a couple of rebuild kits with diodes for the alternators? Or would the protection need to be more elaborate?
If you welded it shut I believe it would be fine. Make sure there are no holes anywhere. Use a good ground as well.
If you want to ensure your bases are covered, I would wrap another one in waterporrf plasic and have it sitting on the bottom of a nearby pond. Like I said, 15 feet of water will do the job.