Thanks for the input guys...
I've changed my plans a bit...I have an Exped pad too so I would only be using my old (very old) Thermarest for this chair. If my Thermarest were to break (which Im expecting it to any time lol) then I'd have to get a whole new Thermarest just for the chair. So I'm going this route:
MEC Senate Chair With one of these for my butt:
Thermarest Trail Seat That way it's a bit more versatile and I'm not having to buy a whole Thermarest just for a chair eventually....