Here's a picture that may help you. I use Micro Ballons and fiberglass epoxy, easy to sand and lite. Put scrap monokote on the area you don't want the filler to stick to, in this case the wing. Then apply the filler to the area that you want to fill in this case the fuse wing saddle. Install the wing or hatch while filler is wet, let set up and pop it loose when dry. Sand area flush with fuse or hatch, remove scrap monokote and you have a very tight seam.