The enclosure doesn't have a way to choose a center point.
You can get the centroid with "Measure > Mass Properties":
I would say creating a sketch about this center and then pulling to a solid is the best method. Before exiting sketch mode, hide the bodies, so that the sketch doesn't imprint on them. When you pull to a solid, select the "no merge" option. Then use the Combine tool to subtract your bodies from the body you just made.
When the enclsure tool works it subtracts the body inside which is the same thing that is done with the Combine tool. If you can't manually subtract your body using the Combine tool, the enclosure tool would have the same problem. Right click on your bodies in the Structure tree and choose "Check Geometry" and fix these before boolean operations can work.
You are navigating away from the AIS Discovery experience