You fabricate the frame to fit your backpack. Measure your backpack from the top of the backpack where the top of the shoulder straps is attached to the bottom of the backpack. That is the frame length you need. Next measure the backpack width, especially where the shoulder straps are attached at the bottom. That’s the width of the frame. You will need to add another cross member between the bottom and the top of the frame. This provides strength and an attachment point for strapping the frame to the rack. After you figure out your frame size you cut the pipe as required to make the frame the proper size for your backpack. You can pre assemble the frame before gluing to make sure you have fabricated the frame the proper size. Test fit the backpack to the frame and the rack. If everything fits and attaches securely you may then glue it all together. I used a Tee fitting at the top corner and made a removable cap so the frame can carry extra water when required.