I was asked by a Thingi user to create a riser for a Perception Pescador Pilot seat.
I have used risers on my kayak in the past but felt that the seat was too high and made the craft unstable.

I've loaded 2 options - 38mm (1.5") high and 50mm (2") high

Disclaimer: I haven't used this specific design so I cannot vouch for it's efficacy on the water but after initial fitting and a bit of rough handling it seems just fine. 

If you decide to use them you will need to use a T-Nut with a longer threaded rod. The risers have been designed so that the original knob and spacer still get used.

The rear of the seat will sit lower than the front so you might want to use a riser there to bring it up to a more comfortable height. There are loads of very quick and easy options available online for this task.

PETG is the preferred material to give the parts some flexibility. PLA can be used but it is not UV stable and is also quite rigid and might crack when sliding the riser over the seat frame.
The test print shown in the images was printed at 0.16 resolution with 30% infill.
I printed them lying on their side to ensure the strand orientation was vertical for maximum strength.