Cargo net, dry bag.
Cheap as heck, nearly indestructible, weatherproof, and simple. It's a bag, you put stuff in it, strap it to the bike and ride. LOL! A 10L would be the size of the smaller yellow/black bag on the tail.
Fwiw, it's what I use to carry my tools, tubes, rain gear etc on day rides in the rugged outdoors. I also wear a small camelback/backpack that I put munchies and stuff in. Cheers!