Bill, foam will not in the least stop a determined squirrel. if a squirrel wants in, believe me he will get in.
The holes he uses for entrance and exit, must be plugged up with metal flashing. This guy will even knaw through wood and try metal fabric.
here is what I would recommend.
get a owl dummy and post him around the entrance and exit areas.
get some red fox urine from the sporting goods store and mist it around the area also. You will not be able to smell this with our weak noses. Bow hunters use this to mask the scent of human. This will really keep him away.
Seems like you are keeping busy, good luck in your new business.