Chris, IMO what you need to look into are the time-tested methods used boatbuilders and racecar body builders.
I have been working with a vacuform vedor for about 12 years or so, Empire West in Graton CA. I have a fair amount of experience with what works, and what doesn't. Large parts like bumper facias or airdams are a non-starter I'm afraid. The materials that lend themselves to vacuforming are not very strong. The tooling cost for experimenting on something that large would be enormous.
Passing the 'wife fit-n-finish test' isn't going to happen with coroplast or riveting together a bunch of plastic or aluminum sheet. Not unless you use those materials to form an foundation, then mix up a bucket of body filler like Bondo, stand back an throw it at the foundation, let it cure, and file/grind/sand away everything that doesn't look like the finished product. Like doing sculpture.
On the other hand, there are plenty of good books that describe the basics of building airdams, fenders, etc from fiberglass, aluminum, steel, but it's not something one just 'dabbles' in. Developing those skills is like learning to be a cabinet maker... it takes hours and hours of actual doing it. Dealing with a little fiberglass is one thing, but living with it for 18 months on a long-term project is another.
There is a great book by Ron Fornier on one-off automotive metal shaping, and some youtube videos you should look at:
20 years ago, 'body kits' for the lower half of cars were all the rage. Fiberglass parts, or sometimes higher-end moulded urethane, were purchased by the DIYer, bonded or riveted on, filled and smoothed, painted to match. I see no reason the same manufacturing techniques that that industry used couldn't also be employed to manufacture boat tails, front bumper facias, and side skirts aimed at better fuel economy, instead of the now out of fashion 'boy-racer/boulevard cruiser' styling.