I was always under the impression that the notch was just cut out of the ones fitted to the original 500s, but I recently bought a genuine 500 lower fin and the missing bit is 100% moulded in, as you can see the mould line and the walls of the notch are perfectly smooth not cut.

Fudge I have a brand new composite lower 500 spoiler for sale for £75, its very well made not the usual tat if its of any use for your project.