"I can't really use the normal U-shape panel clips, because it would be
extremely difficult to raise a panel high enough to slot down into the
gap between two posts. "

Using the usual U shaped ones is easy - if the top ones are just loose
enough to rotate a bit then angle them at about 30degrees and then slide
the panel UP into them and then rotate the panel down and drop it into the
bottom clips.
You only need to lift the panel high enough to clear the bottom clips.

(However, bending the clips - or using the newer ones shown in the video is
probably the way to go now. You would have to lift a panel to ful height
with concrete slotted posts though.