I'm not from the double thickness wall box school (a double thick baffle would be appropriate) but you can get by with a single 0.75" thick wall on the sides, back, etc if you design the enclosure correctly. The secret is to partition or brace the box so that you minimize the wall pressure and length of any unsupported wall. What I do with my line arrays is to partition so that I have full width and depth shelves between every pair of drivers (I'm thinking 4",5", or 6.5" drivers) . For a six foot high box I'll use a shelf every 12 inches so you'll have support within the box. If the box is more than 12 inches deep, I would suggest vertical braces to support the internal panels between each other and the top and bottom of the enclosure. Thus my rule is to have bracing or shelving for no more than a 12 inch width or depth for any unsupported panel.

Bottom line is to design your enclosure like an iron bridge--lots of partitions and braces and you'll be good to go. None of my line arrays boxes don't have vibrational or any box talk issues.

Jim