This could do the trick:

<http://www.bhphotovideo.com/c/product/252211-REG/
Bogen_Manfrotto_635_Quick_Action_Super_Clamp.html>

Bogen clamps work great for this kind of thing. There are adapters and
other goodies that are made to attach to all sorts of stuff.
Non-maring. Also will fit a 5/8" rod in the mount hole. Use 2 clamps
with the rod, a few pipe brackets screwed to a piece of plywood and you
have a shelf!
You may be able to find used ones on Cragslist or ebay

Keith O

On May 31, 2007, at 2:12 PM, Automatic digest processor wrote:

> Date:    Thu, 31 May 2007 12:08:38 -0700
> From:    "Mike \"Rocket J Squirrel\" Elliott"
> <camping.elliott@GMAIL.COM>
> Subject: Some kind of shelf?
>
> When the poptop is up, I'd like to hang a shelf between the top's upper
> struts -- a flat piece of wood, maybe, hung by ropes from a new
> crossmember (piece of PVC pipe maybe) which mounts horizontally between
> the upper struts near the top. I can't quite picture what I'd need to
> mount that new crossmember to the struts. Some fitting that clamps to
> struts, I reckon?