I bought a taillight and strobe that had only seatpost mounts. However, I wanted to mount them on the back of a rear rack (the racktrunk bag hides the seatpost). Thought I'd share my solution FWIW.

Slice off most of an approx-seatpost-size PVC elbow to leave a tab. Bandsaw or hacksaw, then sanding. In my case, my ancient rack has a horizontal flat area at the back. Drill holes and bolt it where desired. A "vertical" tab can be made using a straight piece of PVC. The tab is pretty sturdy BTW.

Have you got a pic of it mounted on your rack with your bag in place?

Here is a photo of my PVC pipe blinkie mount.
http://i143.photobucket.com/albums/r154/n4zou/Hpim0354.jpg
1 1/4-20 bolt
1 1/4-20 nut
2 1/4 fender washers
1/2" PVC plumbing pipe about 1" long.
I cut the pipe with an angle so the blinkie would be just below anything strapped to the top of the rack that would extend over the blinkie.

I haven't decided on fastener size/type yet, but here's one zip-tied in place. Sorry, it's out of focus, but you get the idea. Of course one could attach multiples of these on an aluminum bar or angle or a 2x4 or whatever....

I've wondered about the difficulty for fender manufacturers to work a seat-post sized "bump" into plastic molds of their products so as to provide a mounting point for blinky lights that only come with seat-post attachments.

In another (http://www.bikeforums.net/showthread.php?t=407932&highlight=) thread I showed off my solution (http://i167.photobucket.com/albums/u147/Editz_photos/Mount3_07-14-2008.jpg) that was inspired by this (http://www.krabach.info/bike_light/aux_flasher.html) guy.

Here my Fred mount. A stock DiNotte seatpost mount with a 5mm hole drilled through it. A 30mm long 5mm hex head bolt with a wing nut and rubber backed washer hold it onto the stock plastic Topeak BeamRack mount. Below all of that is a Superflash that attaches to a PB Cascadia fender.

http://i167.photobucket.com/albums/u147/Editz_photos/DiNotte1.jpg

http://i167.photobucket.com/albums/u147/Editz_photos/DiNotte2.jpg

http://i167.photobucket.com/albums/u147/Editz_photos/SuperFlash1.jpg

OK, I have a problem with something. Who decides what is Fred and what is not. Show me the dictionary, or the fellow in the dark corner passing judgment. I don't see Fred here, I see innovation. In fact I have posted pics of some of my ideas. I am not a Fred, in fact, I am the coolest person I know.

You are only a Fred if you let yourself be one.