Here are some illustrations that I found depicting the rucksack in use:
Arrangement of the rucksack contents. This is from the WWII era so a lot of this is conjecture as it applies to WWI.
Landwehrmanner with rucksack- The mess kit was strapped to the front of the rucksack while the Zeltbahn and maybe a blanket are secured under the top flap.
A nice close up from the rear. Most likely, he’s got a greatcoat strapped along the outside in a horseshoe, once again secured by the top flap.
One more view from the year showing how the equipment was arranged on the rucksack.