If you haven't seen this already, (or is it your post on another forum?), it's about all I've got;
http://www.militarymodelling.com/forums/postings.asp?th=128095&p=8
It may have been only on this one vehicle. Interesting that the one photo shows the vehicle arriving in North Africa with this modification, rather than being done later in the field.
I don't appear to have any other photos in my collection.
Osprey's; Modelling the Sturmgeschutz III, covers a build of a Sonderverband 288 vehicle in great detail, but gives no mention of the boxes.
There is some discussion that the boxes might just be empty storage crates slung over the filters, to be removed and used as needed at a later time.