brandon, i am one of those guys on that site, i also race SM, and I have made several bead seats for each formula car i have owned.
here is the deal based soley on my research and what i chose for myself in the car.
the 2 part foam will take up space as you say, but it is not doing so in a safe way. it does not offer impact protection that is available like a bead seat does. I have crashed in both types of seats, and i can tell you that my 2 part foam seat needed to be replaced after it got "used" because it broke.................
I saw a guy who was too cheap/ignorant to buy/make a bead seat in his FF so he used styrofoam from a cooler he tore up. it took up the space. was the dumbest thing i have ever seen. IMO the 2 part foam is the next step up on the evolution chart,it is a custom fit, holds you in place. the bead seat offers impact protection from your own car(seat in the case of SM) hittting your body. this will be secondary to the initial hit. kinda like your head hitting the steering wheel in a regular car wreck................
baldspot seats makes a very nice product. easy to work with, just need 2 people. and a shop vac with ear plugs.
what i would do is make the insert, cut the holes needed for the belts, etc, and then apply a few layers of fiberglass to the back of the insert. this will keep it durable for repeatable uses.
its not hard to do, i dont know why people make such a big deal out of making these.
I say this because i paid someone to make the 1st one for me. it was ~ $300 in labor, and was not done as well as the one i made to replace it.
I use a nomex cover that i apply with a spary on adhesive gotten from home depot(black can). 2 people works best to get a nice smooth cover on the seat. a good knife from the kitchen works fine for cutting the seat. you do not need the expensive eheat knife thing they sell.
lay down cardboard over the holes in the seat so the beads dont go through them. if you dont do this, you wont be able to remove the seat when its dry.
if the seat needs to cut cut into 2 inserts so that it fits, make sure to keep the integrity of the spine area( dont cut the seat down the center where your spine goes)
the info you will get about these seats on a formula car site will be very useful. i am sitting on a gastank, and there are bars right next to my body. the bead seats that I have build were done not to take up space, but to protect me from my own car during an impact.
ask lots of questions, this is a good subject to for you to become an expert in before you start the project.
josh