I picked up a metre of adjustable elastic from Fabricland (you know the ones you've seen in your child's pants with the buttons holes in it). Add buttons holes & buttons to the inside of the waistband.
Iron it over.
Open the button holes.
Stitch the waistband in place.
Insert the elastic using a safety pin to quite it through the waistband. Leave a few holes extra before you cut it, so you can expand the waist as necessary.