Every frame is going to be different due to materials on hand and the shape and needs of your character. The easiest way to create a back pack puppet is to use a hiking backpack with an external metal frame. A hiking backpack is designed to comfortably support 40 or more pounds and comes with a waist belt to provide counter balance to uneven loads.
I scoured our local thrift stores and made a few calls but could not find one. So, I improvised.
An older computer case/backpack provided the basic platform and shoulder straps. The bulk of my frame was going to be made out of 3/4" PVC pipe, but I was worried there may be too much flex in the pipe over a 6 foot length, so I found some square metal bars from a closet organizing system to add rigidity. I cut square holes for the bars and 3/4" holds for the PVC pipe in a number of the large paint sir sticks compliments of Home Depot. I then cobbled together a frame and backpack design. I would really recommend finding a hiking backpack - this took a lot of time and effort.
I incorporated a stronger, wider wood piece to support the shoulders of my character.
I would recommend using the 'T' connectors to create shoulder height crossbars - these ultimately added a lot of stability while walking down the street.