I used to work at a hunting/fishing type store and if I remember correctly, the base they use to fit over the edges of the sled for the seat is pretty basic. They use plastic, but what it looks like is a 2×8 or 2×10 board with 2×2’s channels at either end that fit over the the edge of the sled. It would be simple to make and cost you less than $10. You could also then get 3/4″ pin pedistal base and mount for the seat, but that would add about $20. Of course then you already have the hardware you’ll need for your boat. The board would be mounted across the sled and just drop in over the edge of the sled. You’d need one board for each seat. You could try and mount something lengwise, but at least on my Otter II sled, the front is higher than the back. I think you’d have to permanent mount a board set up that went lengthwise then and I don’t know if I’d want something permanent mounted. Just a couple thoughts. Bret