>>2709888my experience is that by the time you spend enough money to get the pack where you want it, you've spent enough money to get a used purpose built bag that actually suits your needs. it's very easy to find older model quality backpacking bags on craigslist/fb/yard sales for sub 50 bucks.
tinkering is fun though so go learn your lessons for yourself. for 25+ pounds you'll definitely need a frame and waist belt of some kind.
https://www.stitchbackgear.com/articles/2019/6/24/how-to-make-a-kydex-frame-sheet seems like a good starting point?. then add some webbing at the bottom and clip one of these onto there
https://www.gossamergear.com/collections/backpacks/products/fast-belt. or just make your own from webbing but it will hurt like hell probably. 25 pounds it might not make too much difference but if you're getting over 30 you're going to need to be pretty intentional about your load distribution.
