Most minicell foam pedestal seats curve up to meet the underside of the deck at the start of the stern pillar or, more typically, an inch or two forward of the back of cthe oaming. Typically that's all that you'll find in the way of a seat back, although given the low height of most seats, this "back" is still 2-4 inches high. However, I have seen a few pedestals where the foam was continued up at high as the top of the coaming or even as much as an inch higher. You'll have to see what works for you. Outfitting is very much a matter of individual preference. The important thing to remember with minicell is to be conservative about what you cut away. It is always easier to go back and cut more away than it is to try to build it back up if your hacked too much off.
You'll probably also want to use minicell foam to install hip braces on either side of the rear third of the cockpit (once you've got the seat formed, sit in it to determine the hip brace location). Typically these are "L" shaped pads with the long thin portion glued to the underside of the deck and the fatter shorter leg turned up and glued to the inside of the coaming to form the brace against your hip. Another alternative is composite hip braces like those sold by Davey Hearn (see www.daveyhearn.com ).
I used foam bulkheads or "machines" in all the C-boats that I built in the 80s. They are even better than straps for keeping your knees in position, but don't lock your thighs in as tightly as a set of straps, particularly if the straps are high up on your thighs. This can be either good or bad; you can get out more readily if you have to, but you'll find that your butt will lift off the seat more readily if you don't actively tense your legs when you roll or brace. I found it helpful to use the stern pillar as toe blocks to help lock me in when I used foam machines, whereas with straps I don't worry about toe blocks.
Regarding spray skirts, you can get off the shelf C-1 spray skirts already sized for slalom cockpits from either Dana Chladek (see www.rapidstyle.com - starting at $89) or Davey Hearn (see www.daveyhearn.com - starting at $99).
