A couple of days ago , I asked for advice how to trim around a bath vent i installed. Because my siding was not flat, I followed the instructions and cut around the whole perimeter of the vent. It turns out Lowes had a mounting block for this purpose. Although it was too late for me since I already cut the opening, I improvised and used the block. I think it came out pretty good
First 2 pics are before. Third pic is the block I got from Lowes.
4th and 5th pic is the finished product.