MYO lumbar pad help

[spelt with a t]

A search on lumbar pad didn't turn up any specific projects, so here's the situation:
I scored an old external frame pack that I want to put a harness on. I was planning on using aftermarket accessories since my sewing skills are not very good. Straps are relatively easy to find, but the hipbelts I've found all lack padding in the lumbar area. I understand this is because they're made to slip into a pack that has a built-in lumbar pad, but for this use I think I'm going to have to mod a belt to include one. Any tips?

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