they were M548 boxes, a quick Google will get you some images. Often refered to as 20mm ammo boxes, Tamiya do some in 1:35 in their Modern US Accessories set and Quickboost do some in 1:48 for the Revell-Monogram AC-47 Vietnam Gunship.
Here's some shots of the 35SQN Gunnies at Woolshed up in High Range back in the mid 80's. Hopefully the bins are visible. You can just make out the two internal dividing walls in the bin. The big 'Payne' bins for the miniguns were two of these welded together with a detachable pyramid shaped lid.
Although in these shots they are still just sitting on the floor, they normally fitted snuggly on the floor between the pylon frames and were held in place with webbing straps and snap hooks onto the carry handles.
A search of the AWM website should throw up some useful pics.
Hope this helps,