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Hi Karl,
I don't know exactly where you want to bolt or weld the cage on your beetle's bodywork but a thing that is sure, body sheetmetal won't resist any big impact, witch means that bolted or welded you'll probably get the same result. Also it gets really hard to obtain strong welds on a thin sheet of metal, most bodywork is spot welded or mig welded, you'll never see a strong roll cage mig welded, welds are too weak. And Don't try to weld on thin bodywork with another welding process that makes stronger welds, you will probably warp the sheet metal which isn't better. The only places found on bodywork that are resistant are firewalls and suspsension mounts (strut towers) mostly because of their structural design. Firewalls are thick and rigid, it's the same for strut towers, they endure stresses caused by the suspension work. Look at any Rally car and you'll find that their rollcages are secured on strut towers, firewall and if the car is monocoque which is the case for most cars today, they are mounted on the structural parts of the bodywork. Since a beetle has no frame, it's probably considered as a monocoque, try to find out which sections of the body were designed as chassis or frame work, they are the strongest and thickest parts of the car. Seems to me that the floorpan is the strongest part of the car, but we shouldn't underestimate the rest of the it since Beetles had the thickest bodies available on the market. have you ever bent a fender on a beetle, try to pry it out to it's original shape, not an easy task!
finally, on your question on sandwiching (from what I understand) the body between thick plates of metal, it will distribute the forces on a greater area (diminishing the internal constraints in the mounting points) but these plates would only be secured on thin sheetmetal making this still weak.
Frank
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