The great question

...which design would fit the (invisible) rear of the droid.

oliver from england, a well known prop builder and r2 driver for the epsiode 7-9 droids, couldn´t help me to bring light into this mistery...
maybe this droid wasn´t used in episode 9? maybe, when it was used, it was in a cut scene? nobody knows...

so i had some resin parts left on stock to use them on the rear side of the legs...

here are some ideas...

pics will follow