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It occurs to me that it would be cool if I had a real model of a photographers reflector umbrella.

You up for modeling that Paul?

I can, and it wouldn't even be that hard, but I think the bowl prop is fine for that kind of thing unless you actually want to show the umbrella itself in a render.  I've seen a few kinds of those, some are silvered (mostly opaque) on the inside and some are translucent white cloth or plastic.

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