It depends how many layers you have. The landing gear bulkheads in the Cozy MKIV are made of 25 layers of fiberglass all in one layup - they can take the shock of 2000lbs of (loaded) plane hitting the ground at 10G on a bad landing without breaking. There are steel inserts but only because there's a slight elastic effect in the epoxy and you want to put the bolts (studs, really) that hold the gear on "in shear". The gear struts themselves are solid fiberglass and are basically indestructible.
I don't disagree about the risk but PERSONALLY I don't believe this is any worse than for a roof-top air conditioner. Mine has a flimsy plastic shell on it and I've never had a problem with it. If you belly under a low bridge, it doesn't matter what the top is made of - it'll still get ripped off. But a properly made fiberglass setup can take anything you throw at it.
The key is "properly made". You wouldn't expect 22ga steel to take a tree strike - you shouldn't expect the same from 1 ply of BID. You need a thickness appropriate to the task
It's a little hard to tell from your pictures exactly what you want to achieve. But my go-to plan would be to use 3/8" or so foam as a substrate and 2 plies of BID on top of that. I really want to get myself to a point where I'm working on my own (or even HAVING my own) project so I can make a series of videos on this. All the ones I seem to find on YouTube look NOTHING like what I do. I'll paraphrase here just to cover the basics - if this scares you off, stop now. If not, score!
1. Cut pieces of 3/8" or so PVC foam sheet to fit the area you want to fill. Attach it any way you like - I like to use hot-melt. You can get around curves the same way you would with wood - score it, cut it narrower, etc. You can sand it to get a radiused edge. Get the shape where you want it. If you want CRAZY profiles you can use urethane instead of PVC which sands even easier than limestone - you can literally shape it with VERY light passes of 150-grit. This is how I make ducts.
2. Mix epoxy (cheap $30 West 105 with the pump-style pre-calculated dispensers is probably fine) on a warm day to make a half-cupful in a wide mixing cup. Add microballoons ($10) until it's thick - about cold honey consistency. Use a plastic or rubber spatula to spread this on the foam and force it into all the foam holes. Scrape the surface so there isn't any excess - you're just filling the holes here.
3. Put two layers of BID cloth at 45-degree angles on the foam. The micro will help hold the first layer there if it's a vertical surface (hold off on the second layer in this case). Leave excess overhanging the edges - don't trim it now.
4. Mix a fresh batch of epoxy. With a paintbrush, apply it THINLY to the surface to "wet out" the cloth - the cloth fibers will seem to disappear. Work out the air bubbles. A squeegee also helps but be careful not to drag it and disrupt the fibers.
5. Add as many layers of cloth and fiberglass as you want. Two is plenty. Four is overkill but will totally take a strike from an average tree limb. Not that it matters - fiberglass is super easy to repair even in the field with stuff you can store in a small bag.
6. If you want a glass-smooth finish you can apply plastic (3mil works best) to the surface and squeegee again. Leave this on until it cures.
7. If you want to do the back, hit that next. By now this will be rigid so you can pop it off if you hot-melted it on, do the back, then re-attach permanently with a thin strip of BID (4" wide usually) that spans the old-to-new pieces.
8. Peel the plastic if you used it, sand lightly, and apply a surface coat of micro. When that cures, sand to a smooth finish and paint. Alternatively, you can use bondo. Somebody posted that this doesn't mix with normal epoxy coatings - I can assure you that it does, but it sucks so I don't recommend it. (It's heavy as hell compared to micro.)
I will definitely post a video of this as soon as I can. It sounds WAY harder than it actually is. The first layup or two is scary. After that you're like "f-steel, I'm doing it THIS way". The main drawback is cost. For skinning things or making ducts, I think it's about equal and takes less special equipment to do (no welding tools or skill required). But for structural components it costs more to do it right than just buying some C-channel. That's why you don't see many landscape trailers made from it. You could totally DO it, and it would be half the weight - and even the same strength. But AT that same strength it would cost twice as much.