My finished Zaku Mobile Suit. Took one month to build from scratch. Materials made of craft foam, painted with beige elastomeric paint base and regular latex thalo green mixed with beige then overall sprayed a thin surface layer with spray paint Black Board green.
The gun was a solid 2 inch sheet of scrap styrofoam.
Wore it at Cosplay Mania X in SMX convention center in Pasay City, Manila, Philippines.
[link]
But in the event you do have an ample supply of rubber sheets, you may use this tutorial i made to make the zaku helmet base: [link]
As for the details on the costume itself, check out my multiply site: [link]
Album of the finished product here: [link]
Hope this helps. Good luck on your project!