So I decided to build my own mannequin for my Boba Fett costume, since the flight suit is going to be made to measure, and standard mannequins just won't cut it. The mannequin is going to have a PVC skeleton, covered by cling-wrap and duct tape, and filled with bean-bag foam pellets to keep the weight down. I have tried expanding foam, with disastrous results as the foam failed to dry I think because I sprayed too much too fast. Also, it did not fill out the carcass as nicely as I had wanted it to. Hence I went for the bean bag pellets route, which gave me more control.
Here is the skeleton template made out of PVC pipes and T-connectors. Simple straight connections except for the arms, which will be bent using heat gun: