You might want to consider the "space blanket" approach. The thin mylar ones are cheap enough to be considered expendable, and they are super light and transportable. Mounted on something with a little stiffness, they make credible mirrors. You'll want silver to avoid color shifting. I like the spray mounting adhesive that artists use, but in a pinch, you can use grip clips (aka pony clamp) to mount the mylar to your plywood, foam core, cardboard, etc. Whatever you can find on site. Support and aiming may be a challenge travelling as light as you are, but there are options there as well like the mini-grip kits.

Have fun and good luck!

"Logistically hauling mirrors down there is not going to be an option. Going to be in four places in six days scattered around the island, so have to travel relatively light."

I didn't realize you would be in multiple locations. I figured that if you were in one location that they would be able to help you out. Arranging multiple sites to provide you with supplies is not realistic. I don't know what you can do to get a solid backing for mylar foil, but thin plywood should be available almost anywhere and can usually be cut to specifications. Once you are there, maybe you can scrounge the backing and carry it in your vehicle. I wish I could say that collapsible reflectors would work since they are easy to transport, but I have doubts since they have a fair amount of diffusion to their spread.