thanks for the comments guys!
on with the dome
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this may not have been the best way to do it, but here's what I did: I taped the 3 pieces together then started attaching the triangles starting at the front on piece 1, all without being attached to the dome base or the dome form.
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angle 2
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angle 3
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finished and resting on the rest of the helmet.
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the inside view: As you can see, there's just the one piece of tape on the inside, I did all of the taping on the outside. I have since gone back and hotglued the seems, the dome base and the dome form, then removed the tape carefully and smeared elmers white glue over almost the whole outside of the dome.
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angle 2 of the inside
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and one shot of the helmet with dome resting on top, unattached, on the otherwise headless manaquin.