The Viewtainers have a good rubber seal that's good enough for the reactors, and they cheap, 1.99$ easily melted and formed with a lighter etc, a suspended cap is all that needs to be added.

They(Viewtainer) also make smaller ones so you can add one inside the other etc.

No need for a grommet, but if you wanted to make a nice heavy duty pH measurement checker out of Acrylic, which is a material I most often tend to think with, I would have a use for them thar grommets:-)

I might make a nice acrylic version for a client etc.
I'll also need to make them 5 liters of Reference solution also


Regards,
Tom Barr