I took a black umbrella we had lying around and painted it white. I then attached an old Vivitar 2000 flash to the umbrella shaft with some tie straps. I took the bulb out of the flash as I was experimenting with bare bulb flash but you do not need to do that at all. The flash is fired via an optical slave attached to the hot shoe.
I then adjusted the flash to umbrella distance to eliminate as much spillover as possible. As you can tell from the photo it does not spill over much at all.