Multiselect ComboBox, Force it to always display the same text - wpf

Question

I have a multiselect combobox that works fine. Except for the text. I want it to always have the same text ("Commodity Filter") regardless of what the user has selected.
If I set iseditable to true and the text to CommodityFilter it looks fine until the user makes a selection, then it is garbage (displays object type name). How can I hard code some text there? (Actually ideally i would databind it so it can change depending on whether anything is selected, but that would be a bonus at this point)
<ComboBox IsEditable="True" Text ="Commodity Filter" ItemsSource="{Binding Path=ActiveCommodities}">
<ComboBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<CheckBox IsChecked="{Binding IsSelected}"
Width="20" />
<TextBlock Text="{Binding Commodity}"
Width="100" />
</StackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>

Answer 1

I ended up creating a custom object for populating the ComboBox (which had the IsSelected property and implemented INotifyPropertyChanged) because I was creating several comboboxes to control filtering. Once I did this is was trivial to override the tostring on the customobject and pass in the appropriate text. So the xaml did not change much.
I would have preferred to overlay with a text box but that seemed to be beyond my abilities to get a polished look in a reasonable time.
<ComboBox ItemsSource="{Binding Path=ActiveFuturesMonths}"
IsEditable="True"
IsReadOnly="True"
Text="Futures Month Filter" >
<ComboBox.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<CheckBox IsChecked="{Binding IsSelected}"
Width="20" />
<TextBlock Text="{Binding Text}"
Width="100" />
</StackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>

Answer 2

Actually the crux is in setting -
IsEditable="True"
IsReadOnly="True"
Text="Futures Month Filter"
rather than creating custom object. thanks a lot it helped.