Don’t write off using a rear rack just yet with panniers. Have a look at the Blackburn Grid racks - while they’re designed for a full sized wheel they also shift the centre of the rack backwards and the mounting hardware is adjustable enough to work with the lower bolts on a folding bike’s seat stays. While my feet aren’t as large as yours I can manage a pair of large 26l Carradice Super C rear panniers on that setup, so potentially smaller panniers could work for you?

Otherwise, it’s another vote for a saddle bag from me. Depending on the size of your bag and the handlebars you use, you might also be able use it as a bar bag instead if hanging it off your saddle doesn’t work for you.