This one attaches to the float fitting on a Legend Cub. It would be relatively easy to develop a similar underwing tube-type dispenser for an RV. Front of the conformal mount would be a cuff ( a J-hook shape) wrapped up around the leading edge. The tail of the mount would bolt into place using the tie down point. I'd use glass or carbon with a 1/4" thick foam core. Make it about 6" wide, with whatever keel is desired to attach the tube. Lay it up over a wing still in the jig; just cover the surface with packing tape. The rest of the device would be pretty much as shown.
With the Cub, a passenger simply reaches down and pulls the trigger loop. An underwing mount would require electrical control. Rotate the release with a MAC servo or Firgelli linear actuator.
Basic principle is simple enough. It's an aluminum tube with hinged doors on each end. A length of landing gear bungee pops opens both doors when the rotary release is pulled, and propeller blast clears the tube.
We load it off the airplane and then hang it on the mount point. A "safety" at one end allows locking that end closed while the other is open for loading with the tube standing on end. Relatives often like to help, which is fine. There is no muss or fuss, so it can be done in public or in private.
I built this one originally for the dedication of the Tuskeegee Airmen Museum at Moton Field; two of the Airmen had asked to be at rest there. The ceremony was scheduled with a large audience, so it had to be right.
It's important to understand that creamains are not really ashes. The cremation process leaves no ash residue of significance. After cooling, what remains are bone fragments. The cremation staff removes foreign material (usually joint replacements and fracture plates/screws), then grinds the fragments in a special machine. When finished, the standard max diameter is a 1/4" fragment. Most technicians will grind almost to powder, but it remains important that you do not allow high speed contact with the tail surfaces. Our testing (the photo you see above) was done with a mix of pea gravel and sticky athletic field marking lime.
There are lots of ways to address the task, but this approach as been entirely dignified and reliable.