I bought the REI 40L duffle to carry on my SFF PC, but if it wasn't on sale for ~$40 when I bought it, I would have bought this one since it has straps to attach to a roller bag, a water bottle pocket, and internal straps (you may have to get a male-female adaptor to fit them around your computer):
https://www.standardluggage.com/products/carry-on-backpack-travel-bag