Waterfield Designs offers a Surface Go Sleevecase that protects the Surface Go and has room to hold other accessories. You can place this inside of a backpack, or option it out with a strap so that you can use it as a standalone Surface Go case and bag.
The sleeve is sized to perfectly fit the Surface Go, with a small Surface Pen holder right under the flap where it's secure, but easy to get to. You can choose a horizontal or a vertical orientation and pick from ballistic nylon with micro trim or waxed canvas with leather.
When you order, you can choose to add D-rings to add your own strap, or to add a suspension strap to turn this into more than just a Surface Go case, it's a standalone bag.
There's also a slim rear pocket that you can use to stow some small, thin items when you carry this alone.