Dialog button layout: including overlays

I’m new to this group, so I apologise if this is the wrong forum.

I am told 
(https://developer.mozilla.org/en-US/docs/Code_snippets/Dialogs_and_Prompts) 
that I can customise my own layout for dialog buttons, which I have done 
as follows to include a checkbox:

	<hbox id="okcancel">
		<checkbox id="copy" label="Copy" value="1" />
		<spacer flex="1"/>
		<button dlgtype="cancel"/>
		<button dlgtype="accept"/>
	</hbox>

It works as expected, and, among other things, includes the checkbox on 
the same row as the buttons.

However, if move the code to a separate overlay file, and try to include 
it as follows:

<?xul-overlay href="chrome://htmltools/content/dialog/include.xul"?>
<hbox id="okcancel"/>

it doesn’t work properly. The technique works for all other included 
elements, but not this one. Depending on how I tweak things, I might get 
additional vacant buttons, or buttons that don’t activate properly.

Is there an extra or alternative step that I should use for this?

Thanks

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