After reading through what you're trying to do a few more times, I think I may be starting to understand.
I would not run the piece to connect the legs. I was worried about the legs giving out on mine before I built it, but after sitting on top of my box, I felt pretty confident they would hold . By the way, the width of the legs on mine were only like 2" x 2".
Also, when I said before the amount of space between the box and the floor won't make that big a difference in the sound, I take that back. A downfiring box is essentially a ported box. The amount of space that the air between the sub and the floor has to escape would be your port. Too small, and it will sound like crap. Too large, and you lose the "ported" effect, and it basically becomes a regular sealed box. The latter is what you want I think, a regular box, that is downfiring.
The only thing I would warn you about is that if you're doing all the installing and everything yourself, MAKE ABSOLUTELY SURE that the wires to the subs will never fall off. A downfiring box is not your friend in a truck. I've been running on 1 speaker for a couple of months now because I'm too lazy to take my seats out, then pull the box out (which requires 2 people), just so I can get at 1 wire inside the box.
Also, there are a few articles that say downfiring speakers lose their sound quality faster than a vertically mounted speaker, since gravity plays out over time to cause the sub to sag down a bit. Just a thought.
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