Hi,
Sorry for my slow reply. The widget is really meant for mashups more than for showing a single map, but the way you’ve done it should work (‘page_id=’ for a page or ‘p=’ for a post).
I have some other items to implement first but it’s been on my to-do list for a long time to add a widget for a single map ID. Let me know if this is something you need or if the workaround above is sufficient.