I just ran across this sticky and thought I'd put in my dos centavos. On the Squidget, I used a method similar to the simple cleat above. Except that I attached the cleat to the wall, not the floor. When the wall was set on the floor, the side came down beside the floor edge and the cleat sat on the floor where it was attached to the floor with glue and screws.
On the PT, I plan to add another "cleat". Only, I call these 3/4" x 1" high cleats connector strips. One will be attached to the full length of the wall, 1 3/4" from the bottom edge of the plywood skin. The other will be attached to the floor with glue and screws put in from the bottom up into the connector strip before the floor is attached to the chassis (left figure). When the side is set in place, the two strips are joined with screws and glue. The side skin is attached with glue and screws to both the floor connector strip and to the floor itself (right figure). The Squidget cabin was secure with just one strip. The PT will have more holding power to keep the walls attached to the floor. Pocket screws could be used to jon the two strips, but only after the side skin is screwed to the floor connector strip.