I strongly recommend practicing with some fine grained kitty litter first. (Don’t ask why I know this).
In the end I use the paper bag method. Brown paper bag with a line taped securely to the side. Line secured in cockpit, dump bag out window, it comes up short - ripping the bag open. Line length to prevent interference with tailfeathers.
Add flour to mix if the relatives want to see a white “puff”. Otherwise cremains are a bit neutral toned and don’t show up well.
If scattering over water (especially Long Island Sound!) be aware of International Convention for the Prevention of Pollution From Ships, 1973, commonly known as Annex V of MARPOL (Marine Pollution) 73/78. Not sure it applies to aircraft as such. But cremains are of a size that by the convention they should be scattered 3+ miles offshore…