I know that exercise!  My attempts have yielded a big garbage can full of similar results.  
I am yet undaunted in my determination to conquer the foam wing beast.  I just picked up a Tekoa Feathercut system and I am sure that good results will ensue in time with practice.

As with any thing it takes practice and patience to do it.  I still have one of the first foam cutting sets that I got years ago.   Has on-off switch.  Metal bow and .018 music wire.   My first wing was a disaster.  The second worked out okay until I tried to make the lead out holes.   Another learning curve in which I made a jig for putting in holes so I could then core the wing.   Also at one time the lumber supplier carried 2 inch foam.  So I was limited as I didn't like gluing two sheets of foam together.  I haven't tried the heating/air conditioning places yet like some one said as I have too much fun cutting ribs out of balsa.   I think Bobby gets his foam in big blocks.  You might contact him.

Try slowing down for the top.  Home stores use re-ground foam, which has little collapsed beads in it.  All the articles I've seen say to call up a foam distributor and get virgin foam, because it cuts more consistently.

Of course, then you have to develop a relationship with your local distributor, and be content with buying huge bricks of foam.

Contact your local wood supplier and buy a suitable sized piece of Basswood or Poplar. Then you can just whittle one to shape.  Steve

This was my solution. I got some green foam which is more dense, and so better for molds, then I made templates that were 1/8th bigger than the final size. I hot wired them then removed them and sanded down to the final templates with my long sanding board. Much easier than sanding solid basswood. My tube of Sig-ment should be here today so I can glue my balsa sheets together.


MM

When I was hand cutting foam wings I had much better results cutting with the airfoil laid flat, not standing up like your leading edge.
Try putting the template horizontal.  Put a pin into the leading edge center line.  Pin it onto the ends of the foam block. Put some weight ont he block such as a strip of 1/4 in. steel. Start the wire.  Lay it on the pin and pull it towards yourself.  Pick it up slightly as the wire hits the template and pull it around,

I found cutting the foam with a vertical movement was very difficult.  It's too easy to pull the wire off the template.  Nothing provides uniform pressure like GRAVITY.

Then take the excess and tape it back in place.  Turn the foam block over and repeat.  You might find it easier to cut more of the airfoil, say 3-4inches, for more maneuvering room.  File a slight dent in the foam, top and bottom, where the cut off will be. Then stop the wire in the notch for a couple second to make a clear mark for trimming.

We cut several hundred wings this way for both combat and stunt.