When you finish shaping the foam use 150 grit paper by hand to smooth out the roughness. Then take your heat gun and go over it slowly until you get a slightly melted darker grey color. This removes the little cut foam and provides a closed surface. Looks good and removes the debris that leaves a "ring" around your but the first time.

: What you need is mincell foam. It's somewhat flexible and just
: spongy enough, but fairly ridgid and holds whatever shape you
: grind into it. It also is a closed cell foam and it does not
: soak up water.

: Here's a link to what you need for starters:

:
: http://www.nrsweb.com/shop/product.asp?pfid=2076&utm_campaign=shop_comp&utm_source=google&utm_medium=buy_now&utm_term=goog_product_2076&gclid=CIn-sJnwlLECFdOa7QodTC-7hQ

: Take it, trim it to fit your kayak, and then trace your buttcheeks
: out on it. (Please don't post pics of that). Then take an angle
: grinder and grind out a hollow to fit your butt. Word to the
: wise, and this is going to sound silly but I'm serious here-
: don't leave a ridge in the buttcrack area. The first seat I did
: I made it fit my butt perfectly with a nice recess for each
: cheek and it was not comfortable at all. Don't leave the ridge
: up the middle. It's like having a wedgie all day.

: There's really no way to describe making a seat without getting
: into the gory details! Once you get it rough shaped the way you
: want, use sandpaper to smooth it out. The link above has glue
: that you can use to glue it in. You can make a backrest the same
: way, or knee braces, or any other padded item.