I've used hildreth's glass many times for various projects. They've been there for over 20 years. They are on the Port Jeff, Mt. Sinai border north of 25A on crystal brook hollow road. Might be listed as hildreth's auto glass. Not sure if they will actually silicone a box together for you but they have always cut whatever I asked for.
Are you installing the box inside or outside of the tank? If you're going inside, I recommend using black acrylic instead. Clear glass on the inside will end up acting like a algae scrubber.