Build your own. This is my seat in a 31 JC pretty close in size to your Duffy. The complete job was not that closely. The seat is all 3/4 Plywood for strength with metal backrest brackets. The base was a simple 1/2 plywood box with pre-made drop in teak draws.
If you are handy & can cut wood. Tip to tip I would say that seat and tackle box could be done for about $650.00. Keep in mind that I made the seat and bought it to a shop for the vinyl work about $175 with my frame. Making the box was cake. Â½ plywood some mica & teak corner beads. The draws were form Ca. of good quality and less than $200.00 for all three. Beautiful piece, like sitting in your living room & running a boat, plus the seat is wide enough for two people.