ublic class MyModel

{

    public int MyModelID            { get; set; }

    public string FixedProperty1    { get; set; }

    public string FixedProperty2    { get; set; }

    // This is a navigation property for all your custom properties

    public virtual ICollection<CustomProperty> CustomProperties  { get; set; }

}

public class CustomProperty

{

    public int CustomPropertyID      { get; set; }

    // This is the name of custom field

    public string PropertyName       { get; set; }

    // And this is its value

    public string PropertyValue      { get; set; }

    // FK reference and navigation property to your main table

    public int MyModelID             { get; set; }

    public virtual MyModel MyModel   { get; set; }

}