If you're actually seeking a "look" from Rob Roy's active period - say, 1700 to 1720 - here's a short jacket pattern that'd be just right for the common clansman:

https://www.reconstructinghistory.co...ALL&p=416&w=21

For a full-blown chieftain's look, you'd take a coat pattern like this but make it shorter (waist length) to accomodate the belted plaid:

https://www.reconstructinghistory.co...=ALL&p=68&w=21

This is a bit earlier than the pattern Sir William suggested, and more in tune with Rob Roy's era, as opposed to the '45....