I've been wanting to do this also. My plan is to grind off the hinges and any other stuff to be able to make the pan flat between the frame tubes. Then I'm going to try the old cardboard-and-masking-tape routine to make a seat pan out of fiberglass. Then cut foam, wrap it in plastic (water protection) and then upholster with some sort of brown leather.
I have never tried this before and it is a ways off (probably after this riding season). Let me know how yours works out and take lots of pics.