Ant, thank you for taking this work on. You are doing a wonderful job, and have received a lot of good input already.
I think if it were me, I would essentially go the " double vent" route, but I would custom make the inside vents and make them an inch or so larger diameter than normal vents. Aviation snips should be able to get through the material, and you will essentially end up with a giant fender washer with vent holes on the inside. This avoids the cost and trouble of welding.
Plus... i would love to cook on this, and move vents etc, so I would not opt for the wide open option.
No matter what you choose, you can not go wrong. You already rescud this fella and game him a whole new life!