You can attach temporarily first
Some of the commercial WW hip pads have nylon cases and straps designed to wrap through holes in the sides of the seat to hold them in place. Whether you can do this depends on what kind of seat your boat has. These pads also have sticky backing for when you find the right position. The straps alone don’t hold solidly enough.
With minicell, you could temporarily attach them using duct tape. Then when you know the shape, size, and location are right, you can glue them on.