You can support it by using wooden skewers or pieces of cardboard to reinforce it from the inside. Here are some examples:
http://www.405th.com/showthread.php/30811-Liam-s-ODST-Build?p=504645#post504645
http://www.405th.com/showthread.php/33067-Makeshift-support-struts?p=545711#post545711
http://www.405th.com/showthread.php/35492-Halo-3-Recon-Helmet-(WIP)?p=591332#post591332
http://www.405th.com/showthread.php/30811-Liam-s-ODST-Build?p=504645#post504645
http://www.405th.com/showthread.php/33067-Makeshift-support-struts?p=545711#post545711
http://www.405th.com/showthread.php/35492-Halo-3-Recon-Helmet-(WIP)?p=591332#post591332