Depending on how big your backpack is, a milk crate provides a lot of space. You could attach it to your rack using lengths of old inner tube to tie it on. (easy to detach if you need the rack for something else one day) This works great on my 20" wheel folder but not as well on some other bikes due to the rack positioning vs. the seat. Usually folding baskets go to one side of the rack, so you'd want a smallish backpack if you were going to use those.