Flexible fiberglass resin?
 

I would like to make some really custom aero mods, but have been hindered by a lack of materials. Plastic sheeting works OK for some things but is hard to form into compound curves and still needs to be well supported. Sheetmetal has worked for other things but tends to bend after any impact. My local Home Depot doesn't carry coroplast either.

I would really like to make some fiberglass mods, but my trouble is that anything on the front end is bound to encounter a rock, snow, debris, etc at some point and fiberglass resin tends to be a bit brittle.

Does anyone know of some sort of fiberglass resin that remains flexible when cured? A polyurethane of some sort, perhaps? I would really like to be able to make something similar to the material that bumpers are made of.

Ok, maybe this is a little off, but maybe it's something to fill in until you get a better idea...
https://www.rubbnrepair.com/

I think the polyester resins have a little more spring and give in them than the epoxies intially, but they don't tend to weather so well, seem to suffer from UV exposure and die from that instead.

I dunno if you should worry about polyester resins and aging: By now, there are hundreds of thousands of boats made of sprayed polyester/chopped fiberglass, that have been sitting in the weather for decades. So, the stuff is pretty durable, especially since the parts you plan are not structural, and especially since under the car they won't get that much sunlight, anyway. Go for it.

You'll find lots of white and decades old poly/glass boats, because the aluminum oxide used as a white pigment reflects UV.