I used some heavy duty self-addhesive velcro to hold the seat, then it is also slightly adjustable.  It seems to work, at least for awhile. I did lose my 1st seat after a year or two. 

On a somewhat related note, I find it difficult to lift, by myself, my yak onto a car if I have the cockpit cover on.  But without the cover I worry about something (seat) blowing out of the yak while driving. Typically I am able to get help loading/unloading.

   I temperarily mount (Happy Bottom) seats with duct tape for an outing or two to finalize position before making it permanent with contact cement.  I tried using velcro on my thigh support cushions but after getting wet, the adhesive backing does not stick well.

Any tips or suggestions regarding your Shearwater Double build?  My kit is in the garage and I plan to start it in a couple of weeks.  I have reviewed the manual and it appears to be pretty straight forward.  I am considering using a dark stain on the lower three hull panles to give the boat a three colored look.

   I attached my kit supplied minicell seat and custom hip braces to my boat using 4" industrial strength self adhesive velcro. As sticky as it is, the adhesive does not stand up to moisture very well over time. I added 3M Super 77 spray adhesive on my 2nd. attempt and it is holding up very well.  Round over all of the corners, apply a moderate amount of adhesive and stick it. Good Luck!

Thanks for all the comments. Maybe I will start out with velcro until I am sure that the seat and its position are good.

As for the Shearwater Double, overall the build was not too difficult. The instructions are excellent and answer most questions. The biggest problem was to get the deck panels to align properly as they are under quite a bit of stress when bending them over the forms.