I went thru the same problem. Lost over 25 pounds in a few months and my kilt was getting to big. It had three straps. I am not a fan of the third strap. But I did find a way to make my kilt fit. I took of the 3rd strap and I put it beside the strap on the left side. I ajusted the space by aligning the holes. And I put the extra buckle beside the right one to give me more reach. The strap on the inside is not visible and the one on the outside if worn thight will net be visible. When worn bigger, you can always but a belt to hide it if you don't want people to see it. Nobody said a thing about mine.
Here's a few pics.




For my next kilt, I might order it with a third strap so I can do this again. That way, I'll have a strap and buckle that will match my kilt. And not be too worried about weight gain and loss.