Now done the homework I should have at the outset. Original lens on latest models are L794. Just ordered a pair of clear ones for rear indicators which come complete with reflectors as here.

Also ordered a bargain price set of 10 amber coloured BA15s (inline spigots) indicator bulbs from
here so will have plenty of spares. The ones to avoid with offset spigots are BAU15s BTW.
I suspect the OE lens molding may give a more diffused light. We shall see.