Make Support Posts
Even with walls partially supporting the sides, the loft frame needs support. Measure and cut two 4" x 4" posts to the total height of the loft wall (from the floor to the top of the knee wall). Position one post inside the corner of the frame, make sure the frame is level, then mark the top and bottom of the frame on the post. Cut a notch in the post the same width and depth as the frame lumber so the post will fit flush against the frame.
Note: To notch the posts, use a hand saw or circular to make several cuts in the notch space, then remove the material with a chisel.