A rockabilly band that is retro enough to have an upright bass...
Foam in the f-holes, foam under the overstand (which is the fingerboard that sticks out over the body)...basically cram and smother that sucker. It ain't like you need pinky-up woody jazz/classical tone for rockabilly.
I have never seen a band claim to be rockabilly without an upright bass. At least half of the whole rockabilly experience is the aesthetic, in my experience.
JAM