After spending most of the online looking at various rack trunks, frame bags, bigger seat wedges, and handlebar bags, I think that I'll go the opposite of what I was originally intending to do.

I have a milk crate and I also have a beverage crate that is pictured below. If I were to employ either of those, I could use my Knog sling bag and get a laundry stuff sack to handle my layers and anything that won't fit into the sling bag.

The milk crate pros:
-Drop items in and go.
-Mount multiple lights using zip ties.
-Mounts flush with the rack. My new tail light is visible from all angles it was designed for.

Milk crate cons:
-Limited capacity
-Have to remove seat wedge
-Blocks the seat post mounted reflector
-Makes mounting/dismounting the saddle interesting.
-Eliminates the ability to use my Nashbar Townies for grocery runs.

The beverage crate pros:
-It's basically flat.
-Doesn't block the seat post reflector.
-Allows for the continued use of the seat wedge.
-Can use zip ties to mount lights, if done creatively.

The beverage crate cons:
-It cantilevers over the back edge of the rack, which restricts the viewing angle of my tail light somewhat.
-The length also means that I have occasionally brushed my leg while dismounting.
-Also prohibits the use of the Nashbar Townies.
-Depending on the size of bulk of the item(s) placed on it, can potentially block that reflector.

For reference, the bag pictured on the beverage crate is a Banjo Brothers (L=2000 c.i.'s) backpack that was mostly empty and I folded in half. I since discovered that it can just lay flat (more or less) front to back on that crate; it won't even fit into the milk crate.