An easily transportable cotton module builder includes a side mounted dump basket for receiving a load of cotton bolls from a cotton picker or stripper. The module builder is sized to form a module of half standard size, which enables the unit to be readily transportable on fields and roads. The basket expands outwardly, is lightweight and is sized to receive a full load dump from a cotton picker or stripper, then the basket swings up and over a module builder compression chamber. A compression bridge extends across the top of the compression chamber and supports a tamper bar assembly which compresses the cotton dumped in the chamber. The module builder compression chamber takes two to three loads from a cotton picker or stripper before it reaches capacity.