From Craigslist, I give you a $10 Fisher-Price children’s bike seat, rated for 40 lbs. and installed on the tandem in less than 15 minutes. I may need to make a longer bracket for the seatpost stay, because it seems like the geometry of the bike is pushing the seat forward a bit close to the back of the rear seat, but that shouldn’t be too much of a problem. It’s got a three-point harness but nothing for the feet—some of the other models I’ve seen have straps in the footwells, which seems like it might be a good idea. The seller has another one which I may go back and pick up for my mountain bike.