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.

I prefer a milk crate. This looks like the load will be poorly balanced over the rear wheel and is not fully supported by the rack. I mean it's not awful and is probably fine for short trips and certain items but for everyday?- seems not ideal to me.

If you can stabilize the beverage rack, I'd prefer it, because the sides are low, allowing wider loads.

I failed to mention in the OP that either of them is secured to the rack using zip ties. However, neither is mounted perfectly center due to how the 'grid pattern' lines up with the rack. I typically have them shifted just to the right of center (when looking from the back of the bike).

if your smart enough, use a net ovet the soda crate.

FWIW, I'm going with the milk crate (for now). I bought a plasticized laundry bag for a $1, then put the layers that I normally use in that, and then tried both crates with the bag-o-clothes. The milk crate just contains it all better, with room left over for additional or bulkier layers. And the zip ties I had that I used to mount blinkies to before were still there. Had to remove the seat wedge, but I just put in the bottom of the crate and then used its seat post attachment point to secure it to the inside wall of the crate. It bounces around a bit, but won't go flying if I take a spill.

Another reason I opted for the milk crate was I placed both crates on the rack and observed my brightest tail light (that is mounted to the rack) and the soda crate did block it somewhat.

I have a green crate. It's a milk crate, but pretends to be a filing crate, with round holes and ridges inside the top edge for hanging files.

It is secured to my rack with a longish bungee. The middle goes under the rear of the crate, around my rack. This is good for also holding up the red Radian light which I have around the reflector bracket. That way the light shines from the rear of the crate (it was designed to go around the seat post). Then the bungee goes way down the sides of the rack to take up slack, and comes up to hook into the front of the crate. Zipties might be easier, but I didn't have any.

I also have a fanny pack, which was originally acting like a pannier. Now I have it secured to the front inside of the crate. The waist belt extender was making a loop to hold my water bottle in place, but now I have a cup holder I can drop my purse right into the crate with no issues, and even fit a large lunch bag if I put the purse in at a different direction.

So I really like the crate.

I suggest a pair of folding Wald baskets. Seriously, I tried all the other crate hacks. It will be the best $40 you ever spent.

https://www.amazon.com/gp/product/B00...F3XEBXBYNKRV0Y

I've thought about them before, but they wouldn't have worked at the time (my backpack was too large to fit in one). However, now that I've decided to split the load as it were, I'll revisit the idea.

I was going to ask how flush with the top of the rack it could mount; I saw customer images where one had a Igloo cooler on their rack. That is good, as I really want to run with a trunk bag...