I know that there are kits available, but where's the fun in that.

I'm thinking about how to make plugs that require no bracket on top to hold them in place. How much pressure do they need to withstand? I doubt there's much pressure trying to push a plug out. Even the Honda original parts use a simple bracket, but I like the idea of a plug with a central screw that you just tighten a bit to lock it in place - obviously with an O ring at the top. But how much pressure is there underneath the cap?
Sometimes it's negative, so sucking the cap on (that's how the PAIR system works), but when positive, how much positive.
Anyone any thoughts on this?