Check out the belt buckles on this page for some appropriate examples:
http://www.re-enactmentshop.com/p_16th-17th_century.htm
I should point out, though, that with a feileadh mor/belted plaid/great kilt you can cheat with the belt and get away with it, as someone else generally will not easily be able even to see the belt or buckle unless it's a very wide belt and huge buckle. I used to use a German military belt, and no one ever knew.
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