I bought a Dinotte taillight to minimize the chance of getting rammed from behind by a burger- and Coke-juggling kid-scolding mom on a cell phone who's paying attention to everything except driving. I wanted to mount it to my rear rack because I use a saddlebag and don't have the seat post available, but I could not find anything on these fora to indicate that anyone's blazed this trail before. So I came up with a way that, in all humility, I'm pleased as punch with and looks okay. Pictures are below, but here's what I did:
I started with a standard reflector clamp, but turned it around so the straight part that's designed to hold the reflector is bolted to a hole in the back of the rack, and the part designed to clamp around the seat post is pointing up and out. I have an Axiom rear rack on my commuter and a Tubus Cargo on my tourer, and both have the requisite holes. From my local Home Depot I bought a wood dowel that's 1 inch in diameter, and cut off a piece that's about 3 inches long. Length isn't crucial, as long as it's long enough (thank god for that---but I digress). After sanding the cut end, I painted the whole piece black, waited for it to dry, and then wrapped some friction tape around the center to expand the dowel's diameter to fit tightly within the inside of the "seat post" clamp. By clamping the dowel into the center ring of the reflector clamp and then positioning these as shown, the Dinotte fits around the dowel the same way it fits around a seat post clamp and is positioned properly. To top it off---and I didn't expect to be this lucky---the battery pack fits perfectly within the front well of the Axiom rack, where I can secure it with a simple Velcro strap that keeps it from popping out if I hit a bump. The length of the cord is just slightly longer than it needs to be for this configuration, but it's not so long that I need to take up the slack since there's not enough to get tangled up in anything that moves.
All in all, this little set-up turned out better than I'd expected, and didn't take that long to configure once I'd figured out how to do it. It holds the light securely and I can use it with any size saddle bag, with panniers, and even with a small to moderately-sized rack trunk.