Someone told me a while back that it would be McDiarmid Park no-matter who was in it, but I don't know how true that is. Inverness to Perth is roughly two and quarter hours. Dumbarton to Perth is about an hour and a half I imagine. It's near enough centrally. Other options? I assume Greater Glasgow, Aberdeen, Livingston and Edinburgh are non-starters, so Tannadice and Dens would maybe be do-able but then you'd have everyone going east. East End Park would be a good tight venue for the final but it's far closer to Dumbarton than Inverness.