A few photos of the finished repair job. After trimming the corner I rebuilt with a knife and removing nubs sticking through the holes to the inside with a sharp putty knife, I used my little right-angle sander with a 25 grit disc to remove the excess and smooth things out.
Smoothed repair
Rebuilt corner smoothed out
Smoothed repair
Inside left
The new plastic has a lower melting point than the original plastic, so during sanding it would melt slightly if I stayed in one place to long. So I’d work until it started getting tacky, then move to another area while that cooled and so on. Eventually I got it all shaped to my satisfaction. The screened over area still protruded a bit more than I liked – if I had it to do over again, I’d attach the screen to the inside and then fill the hole from the outside. A bit tricky to do it that way, but it would have turned out better.
I cleaned the area to be covered with heat barrier with Acetone and then cut out a section and covered it.
Tracing around the airbox
Laid out on the back of the heat barrier
Checking fit
Heat barrier applied
You can see it’s nice and flat except for one budge where the screen was applied. No biggie – it’ll be be hidden by the exhaust. I remounted the airbox once again to test the fit. So much nicer not to have all of those holes everywhere!
Right side
Top view
Rear view
Now I can finally move on to the next item on the checklist…
ATK 604 Resurrection 
Looks excellent especially with the new heat barrier, I suppose the last priority now would be adding plastic to the inside (though nobody but you will ever see it……). Inspirational to see a classic awesome bike be restored.