Like rocketry airframe tubing, motor mount are also made from a number of different materials. The most common motor mount tubes are cardboard. Many flyers even use motor mount tubes to build minimum diameter rockets for a given motor size. The two predominant types of cardboard motor mount tubes are kraft cardboard and phenolic.
Kraft cardboard motor mount tubes are usually a thin walled spiral-wound tube made from layers of paper glued together in a spiral pattern. Like airframe tubes, most motor mount tubes commercially available for model and high power rocketry also have a smooth glassine covering on them to make finishing easier. Kraft cardboard is the most wide spread due to their low cost.
Phenolic motor mount tubing is basically kraft cardboard that is impregnated with phenolic resins, a tough, durable substance that waterproofs and strengthens the cardboard. Phenolic rockets have been known to land in ponds and lakes and still be flyable once retrieved due to the toughness of the phenolic resin. While having very high compression stength, standard phenolic motor mount tubes are also very brittle and can shatter easily.
While used to a lesser degree, the best motor mount tubing available are fiberglass tubes. Fiberglass motor mount tubes offer the highest strength but cost the most money.