Doing some exploratory work, so I can understand and set up the icon just right, so that it looks "right" when placed on a reverse slope.
I ditched the hand drawn earth and stuff over the bunker in favor of a transparent line, so that you can make the bunker blend in with terrain...
Just needs a bit more work....
Got it to work. New icon attached.
Settings to use in ShpED:
X Size: 88
Y Size: 88
X Offs: 0
Y Offs: 0
Left: -55
Top: -35
Right: 0
Bottom: 33
It really does work like a charm.