cableguy, that's a great suggestion on making the rocks removable in front of the intake! I've already siliconed them on, but a little slice of the knife will free them up again - they're only tacked on at the ends.
BoxxerBoyDrew...ARRGH! I forgot to glue the wood, I only used screws. That's what I get for rushing! I know better than that - I'm sooo impatient! I see what you're getting at, about putting cinders under the box to help support the weight. I don't know that I have room for that, but I can go back and use 2"x2"s around the inside where the top ply and the 2"x8"s meet. I can glue & screw (I just like saying that) the 2"x2"s into both the ply and the frame.
I am getting fantastic tips and advice- thank you!
~ Cindy ~