You ever see one of those cones they put on dogs to keep them from licking wounds? I bet one of those around the trunk of the tree would keep pesky squirrels out of the fruit. It just has to be wide enough the squirrel can't grab the edge and do some death defying gymnastic stunt to get over it. Also, the wide part of the cone would need to point down. I bet you could make one easily enough from some thin perspex. Just measure around the trunk at the appropriate site, and then use that to define the cone shape, with a little overlap so there's no edge to grab for the squirrel.

Thanks!