I always have trouble with wonderflex as a coating for anything - it's one of the more difficult thermoplastics to shape over a form that isn't completely hard and solid, because the advantage of wonderflex - the grid mesh inside that allows it to be heat-shaped in compound curves and folds - is also its disadvantage. It moves around more easily, it takes impressions of your fingertips while you're trying to press and grab and move it, and too much repeated heating and twisting will rip the plastic away from the mesh.
Other thermoplastics like Sintra and Kydex are much easier to heat-mold without needing a mold or form, all you need is a heat gun and you can shape and curve it by hand. However, they have a tough time forming compound curves and complex shapes.
If you don't have a way to vacform large pieces like that (I sure as hell don't), there's a variety of different methods and I think the choice comes down to a combination of budget, workspace/tools, and personal preference. Some folks love fiberglass, some hate it, some simply don't have a safe workspace in which to use it. Some love foam covered with bondo or modpodge or plasti-dip or any number of other substances. I don't, because I think it still looks like foam. Then there's always things like papier mache and plaster wrap, which are good for forming base shapes but need something on top of them as a finishing coat if you want a sturdy, smooth finish like plastic.
There's a lot of options, quite frankly. Look into all of them before making a decision on which way to go. And really, in the end, the under-structure and shaping materials of armor like this may be any combination of the above. What makes the armor look like solid, shiny, smooth plastic with no flaws is your finish - how you prime it, paint it, and coat it. For all you know, you could fool a lot of people just by finishing the top layer perfectly, they would have no idea that the under-structure is foam or papier mache or whatever it turns out to be. This is actually how I plan to approach my Wild Tiger armor - I have some research and experimenting to do before I declare that I will or won't use such-and-such method on each part of the armor.
Founder and President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com