In my home I have a single piece shower / tub fiberglass unit...
something like...

http://www.lowes.com/lkn?action=prod...728-326032AL00

The problem is that the pipe for the diverter tub spout is not
perpendicular with the shower wall, and so when I screw on the spout, I
have about a 1/4" gap at the top of the spout, and the bottom of the
spout is flush with the fiberglass tub.

The spout looks like...

http://www.plumbingworld.com/images/als-1011-2010.jpg

Unfortunately, the spout is one of those cheap ones that is hollow
metal. And so there is only a sheet of metal at the top of the spout
that goes perpendicular to the shower wall, maybe 1/32" thick.

Also, for some reason there is a round sheet of plastic that goes
between the spout and the shower wall. Which creates yet another
complication for caulking. The plastic looks like it is to keep water
out of the hole in the fiberglass that the copper spout pipe goes
through.

[diverter tub spout][round plastic sheet][fiberglass shower wall]

Does anyone have any suggestion for how I can properly caulk this
spout?

My current plan is to:

1) tape the plastic sheet to the shower wall using blue tape.

2) caulk the gap with clear silicone (the gap has no backing because
the faucet is hollow)

3) let the caulk dry (maybe apply a second coat)

4) caulk around where the plastic sheet meets the shower wall

I have now begun to think that maybe I should glue the plastic sheet to
the shower wall, and then do (1), (2), (3), and (4).

Unfortunately this is a new home I purchased from a tract builder and
so I do not have any installation instructions for the tub/shower unit.