For starters you might try using 2" polypro webbing.

For the waist belt you can attach it to a 2" side release buckle that is adjustable from both ends. No sewing involved. You'll still have to find a way to attach it to the pack frame, however. I like to use T shaped nylon plumbing fittings for that.

For the shoulder straps you could sew the tops to the bag or sew a loop in the top and loop it around the top cross bar (if one is reachable on the outside of the pack). Sewing a ladder lock buckle to the bottom of the straps will give you the adjustment capability you need.

Everything I've mentioned so far is readily available on the internet.

I think most waist belts and shoulder straps are overbuilt. I bought a waist belt from Camptrails a few years back that weighed over 12 ounces. That's equal to the current weight of my entire pack.

Want to get radical? I recently made some shoulder straps by sticking 2" strapping tape to a 2" wide X 12" long piece of blue foam. Very light. Haven't used it and it might self destruct fairly quickly. I just throw it out to get your creative thinker going.

I think Thru hiker still sells shoulder straps if you want to go that route.

Best of luck. Making packs can be a lot of fun.