I'm becoming a big fan of literal t-shirt bags. Cut the sleeves off, sew the shoulders so they don't fray, sew the torso opening, do something with the neck hole if needed. It's maybe 5 minutes of work and 50 cents at the local donation store. Now you have a cool, fairly customized bag you can make from various sizes.
Compact, washable, light-weight, strong and cheap.
I mean, they're swimmers, do they have asses? Those abs though...