If you aren't locked in with the Viscacha, the Revelate Tangle frame bag is great, especially for odd shaped things. Spare tire, pump, rain jacket rolled around the pump, and easy access on bike to get to map cue sheet or food. I am a big fan.
Also, I went without a rack, and bought SeaLion dry bags. One for the handlebars, one for under the seat. Worked very well. Also, sneakers are bulky, but flip flops can be strapped right to the outside of the dry bag and held in place.
