'morning sir,
Forget where I read it, but it suggested that the "proper" packing method was to put one's blanket inside the pack against one's back, with anything else on top of that. I've tried that, and my NSK 5# blanket pretty well fills the available space. As such, I tend to put the blanket on the top of the knapsack, and other necessities inside the pack.
I've not seen any reference to the Berdan's style pack specifically related to the gum blanket, but I've found that what works with the double bag works with it. I either tuck the gum between the "front" flap (that covers the pack opening) and the body, or between the "back" flap (that covers the strap adjustments) and the body. The back seems to hold it a little more securely, but also retains heat more. Or tuck it throught the belt, but I don't find this to be comfortable.
It took a bit of experimentation to get the straps that hook under the belt and the shoulder straps adjusted to where I wanted them. I leave one shoulder strap hooked (actually bent the hook closed a little to facilitate this) and only unhook one to put on/take off the pack. Used to be able to hook/unhook it without assistance, but since a shoulder injury I've found it necessary to ask a pard to handle the hookup for me.
If it makes a difference, my pack is one of Paul Lopes/American Civil War Knapsacks - I assume the Frank Packer ones will be similar, but I've not seen one of his.
Hope this helps. Sorry, I don't have any photos handy. If you need them, drop me an email (calum1861@verizon.net) and I can email you some once I find them - I've not figured out how to post photos to the forum...
Calum