Just a quick update on rafthouse situation in Sangklaburi. During high season prices (for Thais) begin at around B1600, and as mentioned in the info for the area, will hold 8-10 people comfortably.
Also, you can go through Kanda, or you can just walk to the area where the two bridges come together and go along a small pathway down the riverbank to the place where most of the rafthouses are docked and arrange for one yourself. There's not much, if any, English on and around the rafthouses, but on the bridge you'll probably be able to find someone who can help you sort it out.
If you've got a small group of people, or if you want some nice quiet time with someone special, these things are the way to go. Stock up on food and water before leaving!