Frog,
I'm sure you looked here in the museum, there's all manor of sideplates. The Mathew Gillespie has an interesting two bolt sideplate. They used all kinds of shapes from moons, diamonds to the somewhat unusual and of course none at all.
I'm kind of at an impasse right now with my 1810 Gillespie. The side panel turned out nice so just the bolt head has a clean look that I find attractive and simple. On the other hand I was planning to do a comet sideplate to commemorate the Great Comet of 1811. I have come up with some designs but I don't know
Really they may be better on the cheek than the sideplate. Incorporating the bolt or bolts and still looking good has me scratching my head on the comet.
I think if you stay true to the overall style any tasteful design from simple to elaborate, one piece or two piece, would look right. I think it's the Gross rifle, nearly the whole side panel is the sideplate in silver. The Gamecock rifle has a nice "winged" sideplate.