Hello Billw12280,
That's a really nice looking knapsack that you have!
I filled my knapsack with acid free paper that my wife had leftover from a project. I put in enough to give it a nice shape. I have it displayed flat with the leather straps on top on a table along with some personal items. Although there are differences in opinions, many collectors and museum employees feel that putting anything on old leather is not helpful and can possible be harmful to the leather. After looking into it more I decided not to put anything on my leather items. Some collectors say they've had good results with preservatives. Maybe do a little more research and see what works for you. It may depend on the condition of the untreated leather? There's a good bit of information to read online. If you're interested, I included a link to my knapsack that I posted awhile back.
https://civilwartalk.com/threads/late-war-double-bag-knapsack.116966/#post-1184646
Regards,
Frank