I got a family member in Germany to send me a basic military rucksack, and that worked well for a long time.

This is basically what it was: https://www.amazon.com/Gootium-21101AMG-Specially-Backpack-Rucksack/dp/B00AV4Y0IS?ref_=fsclp_pl_dp_1

I use a GR1 now but that's more expensive. Pretty much anything that'll keep the weight nice and high, not near the small of your back, and that has thick straight shoulder straps is good to go.