I used some 1/2" thick carpet pad from hardware store. The multicolored kind, made of little chunks of different foam rubber.
It worked very well, a very noticeable difference in sound level. Can just about have a conversation between front and back seat.
My only issue is that the spray adhesive I used is not the best. I used 3M super 77 If I recall. Seems to stay sort of soft, and the foam will peel away in spots with that stuff. A good old rubber cement brushed on from a can would be best.
1955 10hp, 25hp, 1957 18hp Johnsons. 1958 35hp, 1972 50hp Evinrudes. Among others...