Seat and hatch for Necky Spike

Just bought a used Necky Spike. It has no seat and I know I’ll need one. I spend up to 6 hours at a time in my kayak, and need some back support (too old for great core muscles!). Any recommendations from current/past Spike owners?

Also, the bag in the 6" hatch in front of the seat is not waterproof, just a nylon sack. Any recommendation for that?

Thanks for your help and comments.

Lots of choices
Lots of back bands for SOTs will work, probably the Cobra Lowback would be what I would choose.

You may need to adjust where the padeyes are . They are easy to install with a drill, rivet gun, silicone calk and blind rivets.