I'm all for doing things yourself, but by the time you get material, a heavy duty sewing machine and with little or no experience, I think you might want to call Kondo's and get a price quote.

Having your waist belt fail at an inopportune time could be a real pain on a trip.

I bought a GG food pack on sale a few years back, and after getting it, decided I wanted cinch straps out on like my gear pack. Kondo's put two on each side, and for a very reasonable price.  Only possible fly in ointment of having them do it, is this is their busy time?
I'd still get a quote and take it from there ;)