Hi Neil,
I can not imagine what a plumber would require a clear adhesive for ?
When I used to keep tropical fish, the aquariums were made used clear silicone to fix the glass panels together, though transparent, you could still see it.
I’m not a static builder so take my advise on a trial and error basis.
You don’t mention how big the case is going to be, if its small/medium you could try a canopy glue called FORMULA 560 this is used by all modellers to fix canopies and windows. It is white in colour, but dries crystal clear. available at all model shops. If you wait a little while longer someone will come up with the correct method.
Cheers – Kenny
(nothing to do with the thread, before posting, ran the spell checker over this reply – and no errors ! – a first for me!)