Calling all home tailors!

Got a pair of nice Eurostar's, knee-patch only not fullseat, for $29 on E-bay, get 'em here and the waist is about 3" snugger than it needs to be. Final sale. I've done things before like cutting the waistband and putting in an elastic gusset, almost like in a boot, but if you were going to do a major remake on these how would you go about it? I'm thinking maybe splitting them at the side seams and adding a "stripe" of contrasting fabric?

Anyone who's successfully done something like this, would love to hear your method. These will be worn for trail and schooling, not show, and I haven't got much to lose.