Once onboard the raft, how is it that neither the king nor the duke figure out that Huck is the one who put the gold in the coffin? CH 29-33

That is because the king and duke suspect each other of stealing the cash. After the King boards the raft, he grabs Huck, shakes him, and yells at him for trying to get away and for escaping without waiting. The Duke finally intervenes and calls the King an "old idiot," asking, "Did you enquire for him when you got loose?"

Next, the King and Duke get into an argument about the money and start accusing each other of stealing the cash and hiding it, especially since they had added the proceeds of the Royal Nonesuch to the pot.