Multiple marker selection within a box in leaflet

Have you tried something like this?

markers is an array of L.latLng() coordinates

map.on("boxzoomend", function(e) {
  for (var i = 0; i < markers.length; i++) {
    if (e.boxZoomBounds.contains(markers[i].getLatLng())) {
      console.log(markers[i]);
    }
  }
});

I think it could be easy modificating the zoom box that appears when you shift clic and drag in an osm map, but I don't know how to do it

Good idea. The zoom Box is actually a functionality of leaflet.

Here is the code.

Just rewrite the _onMouseUp function to fit your needs.