I would recommend either buying a basic store mannequin and modding it or building one from scratch entirely. It's really, really hard to get costumes on mannequins to look right without a lot of customization. I would stay away from the posable ones. Like a lot of the other posters said, they don't really offer the support you need. I made mine from duct tape and expanding foam. Not fancy but it does the job. One thing you're going to want are anatomically correct posable hands. These guys will tell you getting a Fett mannequin to hold the blaster correctly is a pain. Bottom line, I don't think there is an easy solution. Customization is key.
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