Though not exactly about raft making, there are some entries in A Tramp Abroad that might be illuminating (at least to indicate that Clemens knew his way around a raft). Of course, these are rafts from abroad and it doesn't involve the Mississippi... but, here it is anyway. There are some entries that talk about the actual structure of the raft (not the construction, but his observations and descriptions of the raft) in Chapter XIV ("The connections of the several sections of the raft are slack and pliant, so that the raft may be readily bent into any sort of curve required by the shape of the river"). And, there's also this wonderful entry from Chapter XIX: "The next moment I had my long-coveted desire: I saw a raft wrecked. It hit the pier in the center and went all to smash and scatteration like a box of matches struck by lightning. I was the only one of our party who saw this grand sight; the others were attitudinizing, for the benefit of the long rank of young ladies who were promenading on the bank, and so they lost it. But I helped to fish them out of the river, down below the bridge, and then described it to them as well as I could. They were not interested, though. They said they were wet and felt ridiculous and did not care anything for descriptions of scenery. The young ladies, and other people, crowded around and showed a great deal of sympathy, but that did not help matters; for my friends said they did not want sympathy, they wanted a back alley and solitude." Michael MacBride Minnesota State University, Mankato