I've been trying for a month now to devise a comprehensive rear rack cargo solution using separate, but compatible, components. I bought a new rack for greater canti-brake clearance and a Sunlite grocery pannier last month, and I've been trying to find things to add to these that would serve both commuting and utility purposes. I've also been eyeing this rack bag for a month now and would like to include it if possible.
Problem 1: My grocery pannier has no top cover whatsoever, so its not good in any kind of precipitation and wouldn't allow for privacy (basically ruling out commuting).
Problem 2: The grocery bag just doesn't seem large enough for commuting, especially if I want to stop by any kind of store, say, after work. (so I need some additional cargo unit)
Problem 3: The above linked rack bag is wider than my rack, yet the grocery pannier's stiff back panel is taller than my rack. This means the rack bag would either sit at an angle (not good) or be off-center and extend over whatever side my pannier isn't on.
Problem 4: Cold items should be carried in an insulated compartment, which the rack bag would provide. I don't know of any other solution that would insulate hot or cold items on a bike.
Potential Solution 1: I've thought about installing ONE of those Wald wire baskets on the right side of my rack, letting my rack bag hang over/be supported by it, and using it whenever I have overflow. I could then install whatever regular pannier I have now or in the future on the left side without issue. Unfortunately, a blinky attached to the rack bag would be off-center to the right, and the Wald basket is a little heavy (and an additional expense).
Potential Solution 2: Find some commuting or briefcase pannier that is either level with my rack or has a soft top that my rack bag can push into (unless the pannier is full). This wouldn't require me to purchase a basket and would allow my rack bag to be off-center to the LEFT whenever I have the grocery pannier installed. Unfortunately, I don't know of any commuting or briefcase pannier that fits this specification.
I picked up a backpack at a neighbors yard sale to use as a pannier on my touring bike. I fabricated a frame to hold it from PVC plumbing pipe from the local building supply store. I hold the backpack to the PVC frame and rack with Velcro ties. I have $1 in the backpack, a little less than $5 in the PVC frame, and $3 for the Velcro ties.
The backpack in not modified so I can use it as a backpack if required, all the contents stay dry, provides ample room for items I need to carry, and I can haul extra water in the pipe.
I find that I need the capacity of something on two sides for practical commuting. For a long time, I used a briefcase-style pannier on one side and a grocery basket-style one on the other side (both JANDD). The grocery unit was for picking stuff up on the way home, commuting overflow, lock, etc.
More recently, I got some big double wire baskets. Totally unhip, but extremely practical.