Cut an oval out of craft foam and a matching one out of sticky-back felt. Mine are about 5" x 7". Stick the felt to the craft foam. Cut a small wedge out of both layers.
Close the gap with your fingers and test the fit, seeing if it curves nicely to your head (or your trusty skull-head friend's head). Once it fits, use hot glue to close the gap. If you want to shape it a bit more, hit it with a heat gun for a few seconds until it settles into its curve.
Attach two hair clips to the inside of the fascinator with hot glue. Don't rely on the hot glue, as it will fail at exactly the wrong moment. Just use it to hold while you sew the clips firmly in place, making sure they open and close easily. Try it on, and make note of which side is the front respective to the way the clips want to go into your hair.
I'm using a white carnation from the craft store to diffuse the lights. To make it as flat as possible, pull off the back sepal leaves and clip the stem short. This will make most cloth flowers fall apart immediately, so if this happens to yours, secure the little stumpy stem with some more hot glue.
I wanted a black and white look, so I took a sharpie and colored in all the edges of the white petals. This was incredibly effective.
Now it's time to start decorating. I laid down a layer of netting, then some shaped craft foam and hot glued them to the hat. I added a triangle of fosshape (a felt that hardens into shape when you heat it) to add height and create a space for the battery. You could also use cardboard or card stock for this, if that's what you have on-hand.
I glued feathers over my little battery pocket then slipped the battery inside, making sure it fits fairly tightly. I don't plan to take the battery out (I'll just charge it in place) so I added some glue around it to be sure it doesn't slip out. I added some tubular crin for danglies, and made sure to pay attention to height!
Glue a clear marble or other dainty thing into the center of the flower, then sew it to the Gemma through the unused pads, sandwiching the jewel behind the flower. Glue the flower petals down to the fascinator, making sure you can still reach the reset button and the USB port and battery port on the Gemma. If you went with the capacitive touch code option, make sure you can access pin D0 for color changes.