And that's what we're gonna do.
My bike + luggage setup. Around 60L, and 15kg, of capacity. A bit fiddly on the back, but I've gotten it down to a few minutes of tying and untying the various straps and buckles. I've only used it this way for one 50km ride, and a few shorter ones, but it did the job well on some rougher roads.
My backpack is advantageous for the job, because it has a couple of loops near the back that can be used to hold the base as rearward as possible using the original Brompton bungees, and a couple of loops on the top that are easily attached to the seat with velcro ties – that is to say: this may not work for all backpacks. Lock the sternum and waist straps around the seat post (shortening any slack), tie all loose strap ends together with granny knots, and it's rock solid. Handles really well once you get used to it, too; only takes a few minutes of riding. I love this bike more than any other inanimate object.
by Inu-shonen