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