: 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...

Lots of possibilities....

Minicell foam is quite expensive, and grinding it is a messy and anxious experience for me (you need the right touch with the grinder, and the blank needs to be securely fastened down).

If your plans include a commercial backband, a pre-made foam seat is an alternative. The 'Happy Bottom' seat is pretty comfortable, and has a smooth surface.
You can find it in many places on the web.
http://www.clcboats.com/shop/products/boat-gear/kayak-seats-foam/happy-bottom-pad-kayak-seat.html

Some kayak manufacturers sell seat assemblies- check the Parts/Accessories part of their websites.

If you like working with glass & resin, it's not very difficult to copy/imitate a commercial 'glass seat. I find that some 'hard' seats are more comfortable than foam, and certainly make leg drive and hip rotation easier.