You could (I would anyway) make pedals out of a couple of blocks of wood. Have the blocks cut to the appropriate pedal size. Then drill holes through them just slightly larger than the pedal crank they fit onto so that they'll spin. And, to a width that will allow the crank to stick out the side of the pedal endough to secure it. If the end of the pedal crank is threaded...secure the pedals on with a nut. Otherwise you'll need to come up with some way to keep the wood block from coming off. You could probably use a hose clamp.

Dan