Geo, I also have the seat belt style mount in my car, with enough gap to the net for the safety guy to reach it. I am thinking about changing to a cam-over style latch as I have had my seat belt latch jam before, and if its on fire, I want out quikly. Also with the cam style latch you can put a pull flag out through the net for the safety workers to quickly access.
As far as placement, make it where you can reach it as soon as your belts are undone, but not where you might accidently open it in a rollover crash.
Most of the net manufacturers will build a net to your dimensions, and the important thing the safety guys will insist on is that your arm or leg cannot fit between the net and the roll cage.