Thanks I've been wanting something to do that with for a while; real DB are just too big - my dad played DB; and the first thing he asked about any car he drove was "Can I fit the bass in it?". And he had a much bigger house than I'm ever going to have.
I guess I could try a viola with an octave pedal, but I'm still hankering after something with a sensible fundamental note - partly because certain guitarists think anyone can play bass, and I don't want to give them ideas...