Go_Solo,
I have sent many a person to their final resting place with a C-172. I used to let my instructors go out on that mission, but I took over the mission, since most of the time they would come back with the poor widow's face covered in your late husband, along with the entire rear half of the aircraft.
In order to keep from having their remains permanently entombed in every nook and cranny of the aircraft, I found that 10 degree of flaps, 60kts, with your arm straight out the window at a 45 degree down angle works best. (It also helps to have long arms.) You could kick the rudder as you spread the ashes to give the tail some additional clearance from the ashes. (But be careful at that speed and height being cross controlled. If you are going to cross control, do not have more than 10 degrees of flaps in.)
Happy Hearse Flying,
Capt. Richard J. Gentil, Pres.
Naples Air Center, Inc.