Here's what the final product looks like:
Here's the final product. Depending on your skill, you'll either have a couple small gaps or a few big ones. No matter, it wasn't designed to be waterproof to begin with.
Removal and reinstallation should be pretty easy: the top mounting bolts for the seat frame are removed and the front surface can be slid between the supports and the frame. This requires a little dexterity, but you probably won't want to do it often.
At first, I built the tailbox with my rear fender in place. However, switching from a 700x28C to a 700x32C rear tire created clearance problems: when I pushed back on the seat, I felt the Coroplast through the seat fabric. I found that if you remove the fender, you’d have a little more room between the seat and the tire. The Coroplast on top of the rack and between the tire and seat form a very effective fender. This brought on another problem: I found that any weight in the front of the box caused it to tilt forward and rub on the tire, so I attached the box to the rack with a couple zip ties near the rear of the rack. This slows down the removal process a little, but not by much.
I usually carry my stuff in waterproof stuff sacks, commonly available from sporting goods stores. I have one that carries my spare tubes; patch kits, tools, Powerbars, rain jacket, sunscreen, and assorted other impedimenta. I find that this size box will carry all of this, my work clothes, and my lunch easily. If I need to carry more, I can stuff it in or simply letting it pile high. In fact, one of the dangers of having all this storage is that you forget to empty it out: I kept my lock and rain jacket in it throughout the summer, just because I was too lazy to take them out when I didn’t need them.
Now that you're finished, you can customize your box with reflective tape, stickers, fake fur, or whatever strikes your fancy. Just remember that most paint won't stick to Coroplast- stickers are the way to go.
I've ridden with this box for almost a year, and I can't imagine why I didn't build it sooner. It's just about invisible behind me, except when I hit a good size bump. When that happens, the top thumps on the sides. This was startling the first couple times, but I came to expect it. Otherwise, it's remarkably quiet.