Create Access Rule on entity
Create a XPATH constraint on Mydata.
Click on button owner.
Make an association to the user entity. (MyData_User)
Create a After create event on MyData that fills this association (retrieve current user, change object)
Create Access rule. Put an XPATH constraint on the MyData entity. Click on path to user.
[Module.MyData/MyData_User/User=[%Current_User%]]
With some extension can this method be used if groups of users can see each data, but not all.