Sorry to rain on your parade.

I don't know of anything that, won't over the long term, leave a noticable spot. Just convering an area will create a clean unfaded spot that will stand out. So that only leaves attaching it to places that can't be seen unless looked for.

The under side of the dash comes to mind. See the attachment for a very simplified concept. The red part would be screwed to the bottom of the dash. The green is the base of the pda mount you mentioned. Finding a suitable location depends on the auto.

Another way is to screw through any carpeted area. Carpet more or less self heals when the screws are removed.