There is only one problem with using a shallow eke. If you have taken off some supers immediately before applying the Apiguard, you might well find that the bees will be excessivly crowded into the decreased space. For that reason, using a super as an eke for at least the first stage of the treatment could allow the hive numbers to decline sufficiently for them to manage. You would then need the shallow eke to proceed with the second stage and a shallow eke is about a simple a thing to make, such that no self respecting beek would admit to being unable to make one in a matter of half an hour or less. All in all, despite the super volume being a trifle larger than absolutely necessary, it is not too critical if the ambient temperature is reasonably normal for the time of year, bearing in mind that the mesh floor needs to have the tray inserted and any space above stuffed with foam and the front entrance reduced too with no ventilation holes up top either, so that the maximum heat retention is achieved. At this point in time in particular, it would not disadvantage the bees unduly but leave the Apiguard treatment until September and the situation could be quite different, such that the Apiguard would not work well enough to be effective. Your choice.