that looks like an ABS enclosure(I can't confirm via pic, you'd have to verify), you could get ABS solvent to melt the casing together. I don't know if you'd want to DIY waterproof it, I sure wouldn't trust my skills. You could always drain the tank and take par mearsurements.
How do these compare to the Apogee meters? Seneye? What's the difference between the LED and nonLED versions? Where'd you get them? Cost?