I'm also working on a kilted wedding, so this thread is timely for me.
NPG, with a custom tartan kilt, the black wool shiny button Argyll that you plan on making, and a seal fur sporran, you're most of the way there! I'd avoid the McLeay historic look because it can be a bit costumey unless everyone else is also going to dress in 19th century attire. Add a plaid if you must, but consider the belted 1/2 plaid (à la Matt Newsome) because it will stay out of your way better. Personally, I'd skip it.
For the level of formality you're talking about at an evening wedding, I'd suggest black tie. You could go for the more subdued version, without all the bells and whistles in order to not end up two or three level dressier than your guest. To what you already have, you would just need to add solid colour hose, well polished dress shoes, a low cut waistcoat or waistplate belt (not both, it looks neater), a formal shirt, and a bow tie.
Here's a pic of me in that sort of outfit:
- Justitia et fortitudo invincibilia sunt
- An t'arm breac dearg
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