I've built one as well - I got the motor for free so why not.
I cut up a girl's BMX ($5) and basically just used the rear end. I stuck the handlebars from a little kick-scooter (the type w/ rollerblade wheels) where the seat used to be and flipped it over so it would stand on its own and pedal. I mounted the motor to the front of the seat tube as close to the crank as possible using a couple short chains as clamps. I left just a little bit of the bottom tube for looks, and so I could add a headlight. I need to get a sprocket on the crank is all that's really left to do.
Mine's portable since it mounts to my pocket bike, making a recumbent ebike, and detachable.