My sea kayaks all have the vertical "cheek plates" shown in Nick's book.

I tried unsuccessfully to use velcro on my first build, a Night Heron, but since then I have just relied on having my home made foam seat a tight fit between the cheek plates.

Where the sides of the seat touch the cheek plates, the seat is slightly narrower. That allows for a limited amount of fore and aft adjustment.

I have never had any of those seats move during use or during practice capsizes and since I have to flex the seat quite a bit to get it out past the thigh braces, there is no chance of it falling out. In fact it never moves except when I force it backwards or forwards to adjust for other paddlers.