Question about lighting

Hey, I'm curious about how close I can put a metal halide light to the top of a plant. I plan to keep the plants and lights separate by using a pane of plexiglass under the lights and building a box around them with in line fans to direct the heat away from the grow box.
So basically my question is - if I keep the box with the lights in it very cool and there's plexiglass separating the plants from being able to grow into the lights or get too hot would I be able to keep the light like, 1-2 feet away from the tops of the plants?

An established plant should handle that distance with no panel at all...why do you need a panel anyway?

I just want to lower the chances of a plant getting heat stress or growing into the light and getting burnt. Plus it will allow me to vent the warm, smell-scrubbed air through my house to lower heating costs in the winter.