To most, this Yellowstone bag would appear to be ideal for shopping. Yes? To my betrothed, who is kinda unconventional & easygoing and who chooses not to overthink things that don't need to be overthought, this Yellowstone bag is a gym bag. He doesn't use a gym bag like a traditional gym bag so zippers, pockets, mesh and other hooplah aren't necessary. I didn't want to inject myself into the casual relationship he has with whichever re-usable grocery bag is closest (yikes), but I really wanted to sew a more manly gym-grocery bag that he might or might not use depending on proximity. And so I did.
For the exterior, I used 1 full-width piece of fabric, 22" in length. I cut 2 panels, 18"w by 22"h. You could use fat quarters if you don't mind turning the orientation vertically. With the remaining strips of fabric, I made the 2 straps. The interior is also made with 2 18" x 22" panels.
I used a basic box bottom tote design, with squares in each bottom corner marked at 3-3/4". 3" would have been plenty! Here's a tutorial for making the box bottom like I did, and here's another tutorial for a more elaborate bag using another box method.
The gymery bag is lined with organic cotton/hemp blend canvas. Not a fan of interfacings if I can avoid it.