Get a piece of PVC pipe (100mm diameter, 300mm long) attach a handle to grip it with (cupboard door style will do). Use a couple of layers of masking tape to tape up one end, pour the ashes into the container. Run a length of thin nylon chord around the open end of the pipe and tape it around the outside rim leaving about 500mm length of it free. Now tape over the open end. The chord becomes the method of releasing the ashes. You’ll need to have the person holding the container have it well outside the aircraft before they pull the chord, also slow down - the slower the better. Depending on the aircraft type you could tape the container to a gear leg and run the chord into the aircraft. It’s worth doing a static test using sand to get the method right. I’ve done a number of these over the years. The first being in a Tiger Moth with me flying and my dad scattering the ashes - we learnt a LOT that day. I eventually used the pipe method to spread my dad’s ashes.