I have no first hand experience as I’ve always politely refused to do those flights but have heard the horror stories of others that have.....returned with half the deceased now permanently part of the airplane in tail cones and elevator hinges and fabric interiors.
For an unaltered plane may I suggest a plastic bag with a ripcord? So a practice run or two to get the system right.
And home made contraptions attached to the airplane can get you in hot water with the authorities.
You could also consider a skydiver plane with a rear opening hatch.
After the jumpers have vacated eject the ashes.