I'm thinking that it would be in the form of something like a seat. Maybe it could look like an animal cage. It doesn't matter that much, I mostly care about the functionality (although I of course won't deny good looking things either). You would have to push the entity onto said seat/cage/whatever.

There should be some way to make the entity automatically disembark the rocket, so you can automate the transfer of animals and such.

Of course this should come with a white-/blacklist config for which entities are allowed on rockets.