"In my previous efforts, I've put the can in vertically (right-side up), but since it's a BV 450, it fits down in the small of my back and has worked very well."
Also, you might try putting the BV 450 in the pack with the flat lid against your back. That works best for me.
Bob, you're not gonna believe this, but that's how I used to pack my can in my old Osprey pack. When I started building my own UL packs, I naturally just stuck it right side up within my rolled sleeping pad.
After I posted my original reply above, I began to think about the respective dimensions of the big 3: BV, Bearicade & Garcia. With the exception of the BV 450 (the solo model I have), all are taller than wide. However, with the BV 450, it's actually 8.3" tall vs 8.7" wide.
So, in this case, you're absolutely correct - I should continue placing it flat against my back. And, since it's flat, I can once again place it in the middle of my pack. That is, abutted against my *folded* pad, my pack should be nice & flat through the mid-section.
However, and this is a huge however, in all other cases where the can is taller than wide, it needs to be placed either upright or sideways. Now, this is where it gets interesting: since you want to minimize the length of can running along your spine, it should be placed sideways as shown in the photos @ the top of the thread. And because you want to limit the narrow contact point of the cylinder against your back (even sideways), it should be placed at the bottom as also shown.
OK, the next question is whether to have the can attached externally - really, no different from the external frames that originated in the 60/70s where bags/pads attached from the bottom via cinch straps. The advantage of these models is that the frame is continuous from top to bottom where the hip belts attach.
I think the missing piece of this pack is not having the sleeping pad extend all the way to bottom (to the hip belt attach point) in order to create a complete virtual frame. Other than that, it's a nice re-visit to an older design, updated to deal with bulky bear cans instead of bulky sleeping bags. In fact, I think it would work especially well as the can got taller/bigger ie the BV 500, etc.