: I am at the point to where I need to think about installing a seat
: in my 'Night Heron'. No idea where or what to use. I realize a
: type of foam or styrofoam might be good but what thickness to
: start with? Where to get it? etc...

If you don't like the idea of attacking an expensive block of foam with an angle grinder, there is an alternative.

Use medium density closed cell foam about 3/4" thick. Think of it as building the seat upside down. Glue strips of the foam along the back, sides, butt crack and front. You need at least two more layers on the front than elsewhere.

When you sit on the flat surface of the seat, it sags down between the edge strips to create the same shape and comfort that you would have achieved with an angle grinder. If you sit in it long enough, it begins to retain that shape.

The visible surface of the seat is not marked in any way, so the surface is perfect.

Two things that are guaranteed to give me numb legs are not raising the front of the seat enough and having the sides squeeze in too much on my backside. Avoid those problems and I have a very tidy, comfortable seat.