so I recently bought a rear rack for my mountain bike. I couldn't ride today because it was raining. But I wanted some advice and how to lug my backpack and possibly groceries back and forth. I'm not very wealthy so I'm looking for something of a cheaper value. I was thinking milk crate on my rear rack but its not very attractive. I don't want a pannier because they are very expensive. Do you have any advice for any alternatives I can use or is using a milk crate the best way to go?

edit: if a milk crate is the best choice can it be installed with bungee cord so I can be taken on and off with ease?