Boy, I feel your pain. I was doing the exact same thing earlier this year, going over dimensions and fabric weights and looking for sources.
Here are two threads I posted at the time:
Ultimately I ended up finding some white polyester taffeta that weighs ~1.6oz/sqyd and some Polymax-42 which is coated and weighs ~1.20z/sqyd.
I have been slow to getting my project off the ground...maybe next week? (haha)...and still go back and forth between going light weight with a single layer of 1.6 or a double layer with a 1/4" foam pad. Still undecided.
I like your idea for the suspension triangle. I will not be using cinch buckles, but like the fixed eye idea for keeping the suspension centered.