I know this might be a little bit old, but...
I would be willing to help code boxes and barrels. What I will NOT do is make the models... To do this well, it would quite likely require a change to the cgame, turning this into a full mod... Anyway, if I were to do this, I would make them stackable, and somewhat pushable. While I was at it, I might also make alien and human buildables stackable to a certain extent. (ret on an arm, but no arm on a ret) As far as placing them goes, I would probably partially resurect the !layout command for its !layout edit stuff, particularly the building as a spec pieces...
If this ever gets much further, go ahead and give me a pm, although I usually only check this forum about once a week...