down fire box?

Hey guys, I am getting ready to build a box and I think I am going to build a down firing vented box and I was curious is there a certain distance I should be the box off the ground? From what I have seen in the past they are about 2" off the ground. Is that a good height? I use my trunk a lot between snowboarding and throwing stuff in and around my trunk so I really dont want to take a chance having my sub get damaged and even with a grill I would be paranoid. Any help would be greatly appreciated! Thanks Keith

 the sub should be 1/4 the size of the sub off the floor.................10 inch sub---2.5 inches......12 inch sub---3.0 inches   

never heard of that "rule" in my life. I used to have a downfiring enclosure calculator...got it off the web somewhere...try searchingCustom Enclosure Design

  I heard this "rule" over at  DIY mobile audio.

That particular "rule of thumb" may create the proper space, but the real answer is the exit area between the enclosure and the floor should be at least equal to the driver's Sd  (the "exit area" is the total square inches of the opening you create when you place the subwoofer on the ground, including all four openings if it is unobstructed on all four sides.)  It really is not about how far the speaker is from the ground, it is about how much air can move away from the enclosure.Support the12volt.com

Thanks guys! I will use this info while building my box! Keith