First thing needing attention was to cover the existing vent holes. This will provide an adequate area for the Head. Venting will be provided at another location.

Coat of Bondo and some sanding...

Another application and more sanding. When it starts to look "smooth" apply a coat of primer. It will show all existing imperfections otherwise you may not be able to see. I think it took about 5 cycles between Bondo and primer before I was satisfied with the results.

Now we're getting closer to a smooth finish. Notice that during the sanding a lot of the existing paint came off. That's perfectly OK. Just use self-etching primer (or another good quality primer) when ready. The imperfections in the center are of no concern as these will be covered by the Head. It was a result of something I tried and it did not work as expected ;)

It's a good idea to provide some support in the back. Otherwise, your filler will just come through like cheese through a grader.