It won't work when setting it up in the TE. The "Bas0" is the correct shape to use when setting up the Soft Gun Bat. You want to make sure you have it placed on the same tile with your airfield and that your airfield's .til file has these "Tile Properties" selected:
Is Terrain Cell
Is Square Tile Object
Display Terrain Under Tile
Surface type shouldn't matter, however I don't know since I haven't messed with it.
Now for the TE, after placing your airfield into the TE make sure the shape's "Object Type" is set to "Group Master". Then ensure you select: Field Owned, Can Collide, and Set to Terrain Alt.
All you should have to do to finalize it is make sure that you give it the proper airfield number and zone.
Sorry for explaining it in "simple terms". Didn't want to make it seem as if I thought you were new. Just wanted to make sure you were using the method that worked for me in the past.