When measuring the materials to make a sump, or even when ordering a sump, remember that you are measuring outside dimensions. This is extremely important if you are building a sump around a specific piece of equipment, like a protein skimmer for example, and are very limited on space. If your skimmer has a footprint of 10″x10″, then at the very least the skimmer section will need to have an outside measurement 10-3/4″ x 10-3/4″ if you are using 3/8″ acrylic. This is due to the acrylic consuming some of the space within the sump, making things more cramped if it’s not accounted for prior to cutting.
By the way, Happy April Fool’s Day