If I'm not mistaken, we measured one of the arena packs at 3"center and 2" outer pipes
Sounds about right to me, but keep in mind Brian that you don't have to follow the measurements exactly. Mandos are known for customizing their kits, and I can easily see one having a jet pack specially made.
For the vest, if you don't mind me giving a suggestion; my vest is basically just a front panel and back panel sewn together at the shoulders, with adjustable velcro straps on each side. I made the pattern by taking an old t-shirt and cutting out the flanks.