I can help! First, King & Country released a painted 54mm Black watch WWII piper that is superb:
And Britains also does one, but not as well:
If you wind up buying and painting something from scratch,below are some representative images of Scottish pipers in WWII in various orders of dress. Since pipers typically wore standard battledress, with or without "tribal" variations like the kilt or Scottish headdress, the important dress distinctions are going to be determined by the exact theatre of operations -- are soldiers wearing khaki serge, khaki drill, a version of a parade uniform, campaign rig? A mix of these are shown below.
The regimental tartan for the Black Watch pipers was Royal Stuart, although the "Black Watch" government tartan might be found in some battalions or where supplies were restricted.
This painting is of E.J. Dawson of the Black Watch, during the North African campaign:
This chap, in Italy, is not from the Black Watch, but everything except the tartan is comparable:
Pipe band of the Transvaal Scottish in Italy:
Royal Scots Fusiliers pipers and curious Italian signora:
Cameron Highlanders pipers in France 1944:
Duty piper at the 51st Highland Division HQ, France 1940:
Royal Scots pipers, WWII European theatre in parade dress behind the front:
A Cameron Highlanders piper in the Western Desert:
ponders nirvana.
