Multi-touch surfaces

[ghosstt]

(This all refers to DIY MT surfaces.)
I was looking up information on multi-touch surfaces, and it got me thinking. (Mostly with the camera portion) How deep do you actually need the box to be, in order for the surface to be picked up. I realize its proportional to the size of the surface, but for a smaller surface (6x6), wouldn't you be able to create a relatively thin one? I was thinking more for the use of HID, with a computer already hooked up to a monitor. (Not displaying the screen on the plexiglass) How thin do you think you can achieve?

[windwaker]

touch screen for sale arent thicker than a screen

[jjhammerstein]

Are you talking about surfaces like THIS?
In those cases, it depends on the throw distance of your projector, and then the FOV angle of your pickup camera. It's not hard to figure out how big you're talking about once you have those numbers.