What does Jim build on the raft and why?

Jim builds a wigwam to shelter them from the elements.

From the text:

When it was beginning to come on dark we poked our heads out of the cottonwood thicket, and looked up and down and across; nothing in sight; so Jim took up some of the top planks of the raft and built a snug wigwam to get under in blazing weather and rainy, and to keep the things dry.