Kibler's rifle isn't a representation of a particular school, so I think that any two-piece design would work just so long as it wasn't specific to a particular school - i.e., I won't use one off a Moravian rifle, for example, as those are pretty distinctive. A simple or simple-ish geometric shape, acanthus leaf, or, if you wanted to get folksy, possibly even an very simple animal head might work.
Shumway's Rifles of Colonial America and Kindig's The Kentucky rifle in its Golden Age have a lot of examples to look at.