but can you get to those containers after finishing the dlc?
I'd better hide this--
Spoiler
You can remain in the Sierra Madre after the quest is complete, continuing to explore, gamble, etc. Only one area cannot be revisited. BUT once you exit the DLC you cannot return. Clicking on the main gate (there is storage right beside it) teleports you to a place in the Wasteland (where there is storage), so encumbrance is only a momentary problem. Some of the storage in the Wasteland area resets, though -- use the metal boxes to be safe.
