Here are some choices for building plans.
Bass #1 is simple. Maybe too simple, even for us. We'll need a tennis ball.
Bass #2 looks like the best overall set of plans, and not just because it uses a toilet plunger and an "eye hoe". It looks like the best bass we could realistically build in one night.
Or just go to Tubotonia (The Washtub Bass Homepage) and click on WTB Designs for all kinds of choices and building tips.