The DAK also frequently fitted stowage racks along the rear panel for jerrycans. It was rare for Panzers to carry the oil drums, although you sometimes see Tiger I's and Stug III's on the Eastern Front carrying these large drums on their engine decks for long marches.
Crusader usually carried most of their stowage along the sides, usually hanging from the rail that was fitted to the sand shields - see the photo below.
I bought 5 of Rubicon's German stowage set. I've found about 2/3 of the pieces useful - the jerricans, and oil drums, the wooden crates, and the spare wheels and tracks for the Panzer III/IV. Less useful are the larger, non-specific boxes (basically they're too big), the spare Panther track (not quite the right length) and the SdKfz 251 spare wheels and track (rarely seen on actual vehicles). The soft stowage is nice, but actually better suited to Allied vehicles (like the Crusader!).
I'd like to see more smaller boxes, such as machine gun ammo boxes, and (as you suggest) clumps of water bottles etc. Cutting these from Warlord plastic figure sprues is quite time-consuming, and then they have to be super-glued in place. But I'm much keener to see an Allied set - I have several US vehicles on hold pending the release of this, mostly because I want proper jerrycans.