Installing shower wand on fiberglass surround

We recently moved into our new house and I need to install a wand-type shower in one bathroom.
The bathroom has a typical fiberglass tub with fiberglass surround. The shower arm for the fixed-head shower sticks out of a hole in the drywall just above the top of the surround.
I want to install a wand-type shower -- Danze D464602BN. This shower wand has a tall vertical piece that mounts onto the wall with screws and plastic anchors -- typical drywall mounting technique. It comes complete with a diverter that allows me to use the current fixed head or the wand.
I want to mount this thing so that the mount at the top of the vertical piece is mounted onto the drywall just above the fiberglass surround and the bottom mount attaches to the fiberglass.
How to I mount this thing to the fiberglass?? -- Same as mounting to the drywall -- drill two holes, stick in the plastic anchors, and attach the mount with screws into the plastic anchors? -- Maybe drill holes through the fiberglass and use pop rivets instead of screws?? Seems as though pop rivets would hold a lot better through the fiberglass than scews with drywall plastic anchors -- the part of the pop rivet that's behind the surround should hold a larger surface area then the plastic drywall anchor.
And -- I think I should put a dab of silicone on the back of the mount where it contacts the fiberglass to stick the mount to the fiberglass and to seal the screw (rivet) holes so water doesn't leak behind the surround.
Any suggestions??
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