Transitioning from day to evening

[FossilHunter]

To quote WARPIPER: "Dress is dark jacket and tie. Kilts are encouraged." Since it starts after 6 PM, my take on that is dark suit-equivalent but not black tie, hence the nearest thing I came up with was a black argyll jacket and waistcoat.

In this case your plain black sporran is most appropriate. It’s not even a black tie event and Jock Scot wears a sporran of similar style to those.