Metabolic equivalents (METs) for various activities at different intensity levels can be found on the Compendium of Physical Activities website, but it doesn't list any for shadowboxing, noting only that boxing a punching bag is 5.5 METs. So shadowboxing should be somewhere between 1 and 5.5 METs. This website lists it as 7 METs, but it's unclear to me where that value comes from (it's not in the 2nd and 3rd sources listed at the bottom of the page, and I don't have access to the 1st).
What would be a reasonable estimate?