Originally posted by uccemebug Thanks for the helpful responses, everyone. The idea of a separate light for the background (translucent white plastic -- acrylic?) is a good one.
Whatever sign plastic is made of. Really, just go to a commercial sign place and get as small a piece as you can bend into a sweep.
If you are ambitious, I bet you could figure a way to heat it up enough to form it into an arc.
The light goes below/behind the sweep, and so shines through it towards the lens of the camera. The farther the light is from the sweep, the less of a hot spot there will be, but there isn't much point in losing any light to spill over unless you need some spill to redirect to reflectors.
You can actually do this kind of product photography with one light and reflectors, though you can't colour the light if you only have one.
A couple of umbrellas or softboxes (preferable, as they won't show ribs like umbrellas) coming in from ~75º on either side makes a really easy to control setup. You can either add a tinfoil filled umbrella somewhere to give it a bit of zing, or replace one or more of the softboxes to really make it pop.
Or something.