I've filed the plastic away with a small triangular file.
The first couple I have done were originally in some sort of purple colour when bought, this can be seen where the grey I painted them in has been scratched whilst removing the bulkhead.
needs a bit more weathering before it's finished though.
so far 4 started and 6 more to go!
It's quite tricky to cut the plastic out cleanly all the way down so some extra work will need to be carried out later.