One thing I did on my last flat cap is make a custom toorie (the pompon on the top of the hat). I went to a fabric and crafts store and picked a deep red yarn that closely matched the dominant color of my kilt. I used a method similar to the one illustrated here. Part of my motivation, other than the color, was not having a little round dingleberry on my head. I ended up making it larger than usual, then sculpting it slowly with scissors to be more like the shape of a mushroom cap, and ended up with a broader, lower, flatter toorie, about 1/2 of a sphere instead of a sphere. It looked really good, especially on an antique-ish flat cap, though it would also do well on a more typical Balmoral. Just had to pop a stitch on the liner to get at where the toorie was attached, cut the old one loose and attach the new one, then repair the headband seam. It was about an hour operation, though for someone who knows what they're doing it would be much faster. I'm looking, and I don't seem to have any pics of it, before I lost that hat at an event.