Ok, first cut off the bronze trigger with an jigsaw. Then but the grib piece in an vice and begin to drill holes from the upside .
Start with small size and get bigger till you reached the diameter you need for the trigger. Make as many holes you need one after another.
Always take the new trigger and hold it in position to get an idea how much space it needs to work.
If the opening its big enough you have to take an file to get an even surface on the sides and to remove all upstanding metal.
I know without any pics its not an really good advice but i hope it helps a bit.