I thought about it a bit more on the way home. If I was going to make one from scratch (this one adapted the mesh backpack from the base exchange -- $10 -- and about $2-3 in PVC), I'd use the mesh fabric someone on GC (Karen?) was using to make travel cages. I'd like to use thinner pipe or even heavy-gauge electrical wire to frame it out and I'd put the water bottle sleeve on the outside with just a small hole at the bottom for the drinking tube to enter the inside for the suggies.
As Terry said, it'd be nice to have a catch tray at the bottom. I can wear this on the front side but it would work better with a different strap arrangement.
I had really made this for Anson and Judah (my first two suggies) so I could take them outside and give them more freedom to look around if they wanted but they've only been able to use it a couple times and it was before I framed it out. Laki and Lele seemed to like it enough but were much happier on the drive back in the large popup kennel since it had a frame to hold the backpack and pouch but also had a Custom Cruiser and kitchen with suggie soup in it.