OK, additional pics and approximate dimensions have been added to
webpage.
Renegade: I had thought about using a seatpan for a base but wanted to see how the tailbag would work in general, since a new replacement seat was quite a bit more $ than the bag itself. But I agree the idea has great potential and would make the swap with the real seat a breeze. Using a seatpan will probably raise the bag maybe an inch or so compared to my setup. Also, I think a slightly shorter bag (front to rear) would work better if mounting it to a seatpan. Best thing to do would be to remove the foam from the extra seat first and measure to see what size bag would work best. If anyone wants to sell a passenger seat, let me know as I'd be anxious to buy one to try this idea.