Hi! First of all I have to apologize "google translator" english! ;-)
I have been following your thread with great interest, because I want to build a Boba costume for my 11 year old son.
Everything should be built by myself (except the helmet).
Just like you, I already encountered sintra as a building material.
Sintra is some kind of PVC foam, technically. Therefore it is easy to handle, but due to its softness is not very scratch resistant.
Do you protect your Sintra parts with some special liquid to make it shockproof?
Best regards from germany
Christian