How do you make a big seat smaller?

Seat is big for my bum. Only tip I found so far is putting foam on the sides of the seat to prevent sliding from side to side.

Any other suggestions?

Yes that seems to be the way. I have had to do that when paddling a V10S as i find the wider seat area makes me more unstable as i tend to shift from side to side. I know Epic is now supplying such pouches with different foam wedges to fit different size paddlers (bums). They have made it to fit their V10S but it might work for your ski. Try contacting Greg or Oscar, i am sure they will help you.

I had the same problem with my ICON. always felt as thought I was off balance. I stuck foam on both sides of the bucket which 'Wedged ' me in. Instantly a different boat more, stable,a lot less bracing. Also a waxed seat helps. The problem with sticking foam to surf skis is that they become unstuck with the action of sand, water and just getting into them. What I have done now is cut two pieces of foam the same diameter as a CD and placed them in my paddling shorts either side of my hips. problem solved! no more tape,glue etc.

I didnt use foam but instead use the anti slip mats used generally in the shower floor. I cut 1 rectangular piece for each side (4.5x2.5) of the seat and used, 3m double sided permanent tape. I now fit snugly in my seat, definitely less movement on my hip to stabilize. I can now paddle longer due to better balance and less bracing. I wish I found out about this before my my skwoosh which is now in my car seat! The anti slip mat material also pushes on my glutes which is somewhat soothing while paddling.