The very ratty covers
Twisting a screwdriver under the large cap to remove that one ...
... and levering against that spindle to remove the smaller cap
The old bits, true to the description I gave "The old caps just ping off" one of the smaller ones did just that into places unknown
And the shiny new bits
The new ones slipped onto the spindles with a bit of a push
I cut some concave domes into a piece of softwood with a 'ball' grinding wheel in a drill ...
... which were the perfect shape for the caps. Taped to the wood ...
. and the wood taped to the prop, it just needed a hammer behind and the softwood tapped with a mallet ...
... to pop them into place with no damage.
June 2019: A bang just after closing the tailgate turned out to be a broken spring (A) in the right-hand strut, which damaged the cover. Dismantling showed the central pivot (B) to be worn half-way through, with the hole in the lever (C) significantly ovalled to boot. Neither the spring nor the central pivot are available.
Used strut from Andy Jennings, complete with all fittings, which is handy as the other side has a plain nut on the lower pivot instead of a domed and I forgot to order one with the cover:
Caps and cover removed, lift the outer end of the spring off the pin (arrowed) on the lower strut:
Lift the spring out of the slot in the pivot:
Pivot (left) better than mine (right) so clean that up to use as a replacement as well, but turned through 180 degrees so it is operating against the unworn side:
Pivot hole (top) not worn as much as mine (bottom) either, but chrome shot so reuse mine as-is: