Sewing padding on to back panel?

I would suggest taking that padding off instead of adding more. I have been making my own packs for many years and I do not put any padding on the back panel. Never had an issue as long as I am moderately careful about how I pack. Even my training pack, that I carry on local hikes with 20 lbs of gravel.

If you must add padding, you’ll want some foam that is at least as firm as what you have already, or a bit firmer, so that you definitley eliminate the ridge you now have. For my shoulder straps and hipbelts, I use FY20 from OWF:

http://www.owfinc.com/Foam-and-Padding/products/316/

Cover it with something with some texture to reduce the pack sliding around. I use the lightest Cordura I can get, whihc is 160D, but I don’t see it around lately. Just some Taslan would probably be fine, it has some texture to it, not a smuch as the Cordura but better than nothing. You could also use spacer mesh over the foam, but that adds more weight also:

http://www.owfinc.com/Spacer-3mm-Mesh/productinfo/SPACER3MM/

Adding additional padding may end up causing the pack to become more uncomfortable unless you’re careful. I agree with Paul that you’d want to ensure the padding is at least as firm (ideally more firm) than the middle padding, otherwise it will still feel like it’s protruding out.

You’d probably need to get a couple different pieces that Paul listed to see which one feels the best. Picking them out over the internet is tough. Using the mesh to cover the foam would allow for potentially better air flow, but it also leads to more dirt/rocks getting in there. If it was me, I’d probably cover it with this stuff.

https://ripstopbytheroll.com/collections/waterproof-nylon-fabric/products/210d-hdpe-gridstop