Found sets can be easily manipulated by each user so they are unlikely to be sufficiently "air tight" without more assitance. If you store the user's account name in a text field of each of "their" records, you can use Record Level Access control to limit their access to only records that have their account name in the text field.
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a description of how to set this up.
But won't this show the other User records as No Access and include the other records in counts?
Yes, but that's just the first part. You can set up a script to perform a find. Any find performed by your script or by the user will automatically omit all records that are "no access". And once you have performed one such find, the only way that the user can pull up "no access" records is by selecting either Show All Records or Show Omitted only from the records menu.
If you have FileMaker Advanced, you can then set up a custom menu that either removes these menu options or replaces them with options that perform scripts that show all permitted records or show permitted, omitted records.