It's a one banana job, you should be able to do it.
I used a small sharp cold to make fore cut in the hole, more going sideways then down this will give the bondo something to grab and creates a small dimple to fill. the area need to be sanded down and something need to be used to remove any wax or oil. I use denatured alcohol before filling from the outside.
I used this same method to fill the holes in the roof after removing the rack !
You can go to my build thread page 39 post 777 and you can see what I did, personally I would remove the center ribs as well.
I used a cheap drill stop when drilling out the rivnuts as to not poke holes in the head linner

I would also recommend you do your own sanding and I mean the whole car, It's all in the prep work. Maaco's not going to sand there just going to shoot the pant on, believe me !!
If you do this you can get a decent paint job from them.
If you don't have one I would pick up a 5" random orbital sander, keep it flat to the surface and keep it moving. you can also use it to sand the bondo down using the same method, keep it flat and moving.