Koilife stated the "correct" answer above. Buy a rucksack with a trampoline frame. This is the suspended mesh panel that stands the rucksack away from your back to create an airspace for air to circulate between the rucksack proper and your back. The better brand packs usually feature some interpretation of this design.
If this is not an opinion, my best 'expedient' idea is to obtain a microfleece or microfiber, terry-cloth flocked towel. Get one with the thickest terry fleece flocking available. Try your local auto repair and supply store for a "detailing" towel. They usually come in small enough sizes to do this. it only needs to be about 12 inches wide by maybe 16 inches long to cover your back where contact is made. I believe a common size towel is 16 x 24 inches. If this is the case, fold the towel in half lengthwise to double the padding and wicking capability against your back.
Use medium to large, safety pins to attach the corners, top, and side edges of your towel to the back panel or should straps (where convenient) of your rucksack, where it contacts your back. Experiment to find the best attachment solution for your situation. The terry cloth towel will provide padding, wicking, and some ventilation. As a bonus, the microfiber / microfleece towel can be used for bathing on arrival. It can be easily washed, rinsed and dries fast.
I hope this helps.