Been looking through them too most can be had for as little as a £1 each, on site there are ultrasonic probes to measure depth of fluid etc they have a shade over them to protect them from the elements, they look about perfect
I had a tank in for a few add ons a while a go lovely shaped well made thing radius corners etc it had one on it, opening was 150mm, the sides weren't angled straight like a dog bowl but spherical, around 60_70mm deep with a 80mm diameter bottom
I never worked it out as I just went of his dimensions, there's things I advise when making them but most times it's out of my hands it was 840x520x175 sloping up to 50mm