"Fiberglass cloth" is sometimes called "cloth" and sometimes "fiberglass" by lazy people like me who prefer single-syllable words.
You can bend the foam core before you do a layup on it. After that, you can park a car on it and it won't bend. Fiberglass is incredibly rigid, especially on a core. There's a story that supposedly the wings of the B-42 bomber (aluminum and steel ribs) droop 1.5 feet at the very wingtips. Remade with fiberglass they'd bend a quarter inch (or something like that).
The standard process is to put the foam where you want it, and shape that with sandpaper, cutting tools - whatever. Get it exactly the shape you want, which is easy (which is why people who like this process... like this process in the first place). Then do the layup, and it keeps that shape forever. You can make curves, angles, join multiple pieces with 5-minute epoxy to make complex shapes, etc. When making things like ducts you can remove the foam core later if you cover it with duct tape before doing the layup - epoxy won't stick to duct tape. For others, you leave the core there and it provides additional rigidity and crush resistance.
You can crumble 3/8" PVC with your bare hands, and shred raw BID cloth the same way. But once laid up (just two layers) and cured, you can walk on both without making a mark. (Lightbulbs going on in my head - I just realized how I want to make my roof deck.) If you make a structural shape like an I-beam and use a few extra layers, you can park a car on it. There's literally a math formula you can use to determine its strength, determined by the type and weight of the cloth you use.
* Epoxy. The $30 West kit from hardware/boat stores is fine if you aren't building an airplane. Get their pumps too, it's like a $12 pair you stick on their cans that dispense pre-measured amounts.
* Rubber squeegees. In a pinch, you can cut one from a coffee can lid - that soft plastic is about the right strength. Round the corners so they don't drag. It needs a smooth edge.
* Mixing sticks and disposable paint brushes. Get the Harbor Freight "chip brushes". Don't even bother trying to clean them.
* BID cloth. You can get all kinds but anything that says "BID" is fine if you aren't building an airplane.
* Disposable scissors. Well, these don't have to be disposable, but fiberglass will dull a cutting edge very quickly. I like the Harbor Freight shears they sell for like $1.99. They last a few months.
* Mixing cups. Wider/flatter are better.
* Microballoons. A 5lb bag will last forever.
* Dedicated hair dryer. Optional but VERY helpful. You want the epoxy to spread smoothly and evenly, and heat helps it along. A heat gun is too much. This also lets you work when it's colder out. You WILL get epoxy on it, so don't use your wife's! Walmart, $10.
* Nitrile gloves. Epoxy is sensitizing! Never touch it. It won't kill you, but you will get a nasty rash, it sucks, and there's no reason to have that happen. If it doesn't happen the first time, it will happen the tenth, or the hundredth - but it will happen eventually, guaranteed. Wear gloves and you'll be fine.
* 3mil plastic. Anything will do, but it's best if it's full width and isn't folded. I buy mine for like $12 a (50yd) roll online from some shipping company. (The folds in the ones from Home Depot make creases in the layup).
* A dremel or (better) Black & Decker rotary tool. Get a few dozen sanding drums and the little raspy cutter thing - the one they sell for drywall.
* Some hacksaw blades. Hacksaw not required. You can make amazingly straight and clean cuts in fiberglass by just hand-pulling one of these blades along a line or straightedge.
Sounds like a big list but a lot of people have most of this stuff lying around. Most of the money goes into cloth and epoxy. For what you're trying to do, with careful shopping, you could probably do it all for $200.
I'm not specifically trying to "pitch" you on doing this project with fiberglass. Plenty of people hate it. But if you DO want to do it, I'll post here all day long about how to get it done. At least I can contribute some value to the forum.