Many of Shakespeare’s plays contain scenes where characters listen in on one another. Hamlet (where Polonius gets killed doing it) The Tempest (where Prospero checks up on his daughter doing it) and Love’s Labour’s Lost (where four characters do it to each other all at once)! As we see from this list this gag can have comic or tragic consequences.
In Much Ado About Nothing it is used for comic purpose where the men make sure Benedick can hear them whilst they talk about how in love with him Beatrice is, and the girls play the same trick on Beatrice letting her hear their talk of Benedick’s love for her. On the modern stage I have seen this done in numerous comic ways…
Benedick climbs a tree and falls out, or gets watered, or both! Beatrice hides behind sheets, a tree on in plain sight on a scooter whose hooter she sets off. But the most comic version I have ever seen was years ago at the Barbican when I was a school girl – here Benedick was completely concealed in the arbor but his reactions were measured with perfect comic timing by puffs of smoke from the cigar he was smoking. But it is likely that Shakespeare didn’t resort to any of these comic contrivances.
On Shakespeare’s stage you really don’t need any props to make this scene work as you already have those handy pillars. Making use of something which many directors (not to mention audience members) today would find a major inconvenience, Shakespeare shows his ability to adapt to the limitations and advantages his theatre offered.
If he did use any external prop to dress this scene it would have been a simple trellis arch which could represent the arbor in which Benedick walks whilst his friends trick him. This would be simple enough to carry on and carry off if need be but it could equally well be a permanent feature on the stage for Much Ado About Nothing. Something of this kind was probably used in Twelfth Night where Malvolio reads the trick letter whilst the others watch from the bushes.