What glue for rubber seal on SPL Housing

[Jubes43]

Hi Guys,newbie here. Was hoping for some advice regarding glueing a circular rubber ring that is between the grip and the housing? it's an old EOS 3 housing that hasn't been used in years due to having kids and no life anymore. Got an EOS 5D III but unfortunately the housing is not deep enough so can't get lens ports on...gutted!!! Please help urgently as I leave for El Salvador on Sunday and really wanna take some water pics.

Thanks