I used to use a seal line baja bag. It is a dry bag with straps on it which is not for everyone. It holds a lot of stuff (30 liters) and has worked fine for a while.

Next week I am going to buy a seatpost rack and a trunk. It has a quick release so it will be quick to get on and off but can still hold up to 20 lbs. If I carry big things I will supplement with a backpack.