Made my own
Using some PVC pipe and aluminum bar stock, I was able to make one in a few hours. Maybe this will help in your mounting ideas.
<table style="width:auto;"><tr><td><a href="http://picasaweb.google.com/lh/photo/SNew3s-IRyJT3cedobtwrw?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh5.ggpht.com/__pxwMbtpK74/SomYfq3lGyI/AAAAAAAAEg8/47OT_yuVdQ8/s800/IMG_0053.JPG" /></a></td></tr><tr><td style="font-family:arial,sans-serif; font-size:11px; text-align:right">From <a href="http://picasaweb.google.com/greg.hilchey/Stuff?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite">stuff</a></td></tr></table>
The cap. Can easily add a locking latch or cable.
<a href="http://picasaweb.google.com/lh/photo/3VXL9gx1xERqxrb-jZIfUQ?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh5.ggpht.com/__pxwMbtpK74/SomYfxNYVBI/AAAAAAAAEhA/kKVNepakSzk/s800/IMG_0054.JPG" /></a>
A view from under. The aluminum was bent, cut, and bolted to the pannier mount.
<a href="http://picasaweb.google.com/lh/photo/J_c17QLM3SyAuvVbdCVH8g?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh3.ggpht.com/__pxwMbtpK74/SomYgbqkjQI/AAAAAAAAEhE/egIKtiA7aEQ/s800/IMG_0055.JPG" /></a>
A view from the front.
<a href="http://picasaweb.google.com/lh/photo/ZCyN8x-qz7bw83PgVbnSvw?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh5.ggpht.com/__pxwMbtpK74/SomYg1GoxLI/AAAAAAAAEhI/-yPDJcwhdXY/s800/IMG_0056.JPG" /></a>
under from the front.
<a href="http://picasaweb.google.com/lh/photo/i7NeYhVwiVx1fAZUbf85cg?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh4.ggpht.com/__pxwMbtpK74/SomYhYIRQrI/AAAAAAAAEhM/hNI-PrhOsx8/s800/IMG_0057.JPG" /></a>
side view with the pannier removed.
<a href="http://picasaweb.google.com/lh/photo/e8MX-0gWgAytymsRLBltRA?authkey=Gv1sRgCPHR35z10JPA7AE&feat=embedwebsite"><img src="http://lh6.ggpht.com/__pxwMbtpK74/SomYiNXA2OI/AAAAAAAAEhQ/Rw9HgzakTOg/s800/IMG_0058.JPG" /></a>