AlanB,
I haven't used a PHD so if this isn't helpful I apologize...
Rather that rigging something to hold the PHD up, have you considered something trying to use a ground level device? Use your largest auger bit to bore a PH in the general area you want to store the digger, enlarge it slightly (with a shovel, pick, sharp stick, etc), place about 6 inches or so of gravel in the bottom (to aid draining water) and sink a section of drainage culvert (larger than the PHD in diameter of coarse) into the hole. I would recommend extending the culvert about 1 foot above ground level and painting it safety orange to help avoid accidents.
This would keep the attachment low to reduce the possibility of straining to position for hookup and almost completely eliminate the possibility of it falling over while stored.
Again, I haven't used a PHD, but it has been my experience that items stored lower are less apt to injure than those held up high.
Of coarse this can't be done inside (my wife would kill be if I bored a hole in the garage floor /forums/images/graemlins/frown.gif .....
Hope this helps,
KiotiOrange