I put helium in a balloon (with a maximum pressure of 1.5-1.7 atm), but I do not want changes in the volume, so I have to use a rigid material (non-elastic). To make sure that there is only helium in the balloon, I have to create a "strong vacuum." My fear 'that, in the material can break or deform. Can you recommend a rigid material (not heavy) that resist also vacuum ?? I have think at PVC, but i'm not sure that is a good idea... Please Thanks!