Amethyse wrote "What are we supposed to look at then?"
It's a dilemma, isn't it. As serious, po-faced musicians we want our audience to focus on the music, not the visuals, but we chose an instrument with a powerful visual aspect - the musician doesn't touch it! So people are going to look whether we like it or not. Which means we should consider what they are looking at, and how it reinforces our music. For me this is simple - I'm pretty much old-school Industrial slash DIY ethic, so the more battered and used my etherwave looks the better. Which is convenient. I like convenient. ;-)
But perhaps you have a strong 50's retro vibe going on. In which case you want this case:
http://www.pureimaginationco.com/theremin.html
Or if you're an old hoofer from the music halls who gives spirited performances:
http://www.flickr.com/photos/8666613@N04/2263367827/in/set-72157603907186727/
And of course there are those amongst us who defy explanation and require the services of a taxidermist:
http://www.nervoussquirrel.com/badgermin.html
I could list loads more interesting enclosures, but three is enough for now.
You know, for someone who is so inclined, there is probably a coffee table book to be written; "A Pictorial History of Customised Theremins". :-)