So in the photo we have vellum on the left, mylar in the center and tyvek on the right. I measured the light with a studio grade light meter 2ft away from the source.
Vellum = 3.5 grams, packs well and can get soaked if left to dry while folded, can be refolded if crushed but you'll probably want to make a new one later.
Mylar = 6.1 grams, slightly brighter than vellum, can be used in rain, doesn't fold flat on its own, can take a beating but destroyed if crushed.
Tyvek = 2.6 grams, sits completely flat, water and crushing no problem but only half as bright as vellum.
I forgot to mention after you have folded the cube, blow it out then cut out the bottom from the hole. Leave the triangular flaps to use as mounts.