It makes no sense to pay 40 dollars for a pair of boxers and throw them out in 6 months because of a stretched waistband. Doing all the work to put another waistband on every 6 months does not make sense either. I want to add a drawstring but I'm not sure what material to use.

The patagonia boxers are made out of Tencel which is branded Lyocel. The two problems I have is 1, which material is good for a permanent waistband with draw string replacement?and 2. where to source it from.

Here are the boxers for reference -