Those are good looking sound holes and I think they would work well I will try that shape on my next build if you don't mind .
I visited a nyckleharper builder in Sweden some years back, the traditional ones I had seen up to that point had diamond shaped sound holes but for his builds the 'f' hole shape like on a violin was being used. I asked why he had moved from the traditional shape, he explained that the longer f shape hole to ether side of the bridge allows the soundboard to flex more thus increasing the movement of the air inside the instrument and hence the volume. The effect they would have on a mountain dulcimer might not be so pronounced but is worth a thought.
Neat. Someone posted an analysis on here recently talking about how soundholes that are long and narrow are the loudest. Maybe these long narrow soundholes are the ideal!