I've got several mannequins an have suited them up in my 89 and Begins. The problem I find is that most of them have really long legs, which is somewhat of a pain. Of course, I fitted my suits to me, and it's hard to find a matching mannequin. There are some really good sellers on Ebay from New Jersey, I believe, where I've picked up a few. The best one so far has copper pipe elbows, so you can pose the arms slightly. Perfect height and works great for my '89 suit. I found a big muscular mannequin for the Begins, but even with the larger shoulders, the v3 armor shoulders are still wider and the biceps don't sit properly on the arms. The rest of the body works perfectly though.
The most important thing is to make sure there's LOTS of room in the armpit area. Many mannequins with their arms down to the side don't leave any gap, and the undersuit + armor usually ends up being very tight, even with a bit of a gap.
Here's a bad camera phone pic from the night I put it on the first time. You can see the arm problem, and because it's 6'4" the torso rides higher on the body than it does on me, showing the gap between leg armor and torso. In person, it's not very noticeable though - the camera's "enhanced" contrast pics up weird lighting.
(And yes, I know those boots aren't supposed to go with Begins. They're placeholders until the UDR boots arrive.
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