They are triangular or square

 Henry Baldwin Ward & George Chandler Whipple, Fresh-water Biology, New York, John Wiley & Sons, 1918, pp.900-901.

The most familiar larvae are the well known «  caddisworms » that construct portable cases (Fig. 1371), in which to live, and carry them about on their backs. These cases are made out of a great variety of materials : sticks, small stones, sand grains, bits of shell, of leaves or of bark ; in short, almost any solid material suitably small and available. In many species the construction of the cases is so uniform in pattern and materials that the larvae may be know by the houses which they drag about. The larvae of the Phrygeaneidae construct cylindrical cases made of bits of stems, grass, etc., placed lengthwise in a  continuous spiral band ; the larva of Helicopsyche builds out of sand grains a spirally coiled case, shaped like a snail shell. The materials of the case are always stuck together by means of the secretion of the salivary glands. Usually the cases are cylindrical but sometimes they are triangular, or square in cross-section. Usually the sticks used are placed lengthwise, but sometimes crosswise, as in sticks chimneys, to make the bulky and cumbersome dwellings of some of the Limnophilidae. Sometimes, on the other hand, they are constructed so light and thin as to offer little hindrance to free locomotion, and a few larvae with well-developed swimming fringes on their long oarlike  feet swim freely about.