It's fairly rough, but maybe it'll inspire somebody.
Step 1: Custom fit for your head
Step 3: Nose piece
Step 4: Let the shaping begin
At this point I had to just sculpt till I had results I could live with. I used wadded newspaper wrapped in masking tape to get the shapes I needed, and then taped them on. Using a snap-blade style utility knife with a sharp blade fully extended, I trimmed off extra bulk as needed.
I had to put about a six-inch slice in the back of the mask to get it on and off.
The antennae were made of cardboard with newspaper on the inside to add bulk.