Highlighting edges of a sprite group?

I’m using a sprite group to wrap a handful of sprites and I cannot seem to find an effective way to highlight the edges as shown in the example image. The boxes will vary in height.

Any ideas?

No thoughts on how to add a border to a sprite group?

A post process maybe? is the grey area in the middle “open”?

Yes, the gray is just the background/empty. You cannot change the material on a sprite group some reason.