I've got the 40L REAX Poseidon Dry Bag https://www.cyclegear.com/accessorie...sku_id=1643869 I took that on an overnight with a lady friend. It had enough room in it that I gave her one of my saddle bags, used the other saddle bag for snacks/drinks/tools/flat tire equipment. I was able to fit two days worth of cloths, my CPAP machine, my drone, and toiletries in there with some room to spare. I bought it on sale for around $68, haven't taken it through any rough weather yet, although I'm planning some long trips to Utah/Colorado in the next few months so I expect I'll need the water proofness.
Edit: It fit well on the backseat and on the rack.