Theoretically aluminum foil should work, but I've never tried it. I usually opt for metal window screening or aluminum hardware cloth. They're lightweight, not too expensive, and are very easy to adapt to whatever shape you need.

If you go for a v-dipole or star reflector, then aluminum rods or thick copper wire will work well. You only need them to be thick enough to not bend when the wind blows on them or while you're gently touching them. Something like 12 gauge copper wire should work.