How to hide parts of prefab attached to trackable object

Hi I’m pretty new to Unity AR so apologies if my post and title are badly worded.

I’m trying to replace an image with an open-sided box, so the user can see something below the plane of the tracked image. This seems to work fine, except that the user can see the sides of the box, and I only want them to be able to into the top of the box.

The image shows the box attached to the tracked image. The top of the box is at the same height as the image (although it may not look like it). How might I go about only allowing the user to see into the box through the top, so they do not see either the sides of the box, or indeed the contents of the box from the side? The size of the top of the box will eventually be the same size as the tracked image - I’m just trying to solve the above issue first.

Thanks for any help given - much appreciated.

As this a more general “How to” Unity question, rather than specific to AR, I’m going to move this thread to our Editor & General Support forum.