This project was part of an undergrad design class
where we wanted to make cheap bicycles more utilitarian. My goal was to make something that you could produce in a dorm room, but would hold 3 people.
I used a 3 speed, gear-hub, girls bike, but you could probably adapt the simple design to work with any bike. The end result is pretty goofy looking but very effective at hauling people and cargo. You can find a bike like this for $15 at goodwill and should plan on spending $10 on the bolts and wood.