I put together a very small example to show how building can work. It does very little but should give a decent idea of how the general structure is: define some functions, then declare that you provide a certain package.
It currently only works with the version of Ecaml on opam (v0.9.0), and relies on the executable format being a valid shared object (seems to be true on macOS, and I think Linux (ELF) too).