i'm not BB, but i'll take a crack at part of this...

first of all, he has a better shadow catcher that he gives away.  i suggest pointing at that instead of the ground  plane one.

second, you can fake it.  you've already got the ground worked out, now all you need is the sides and top.  if you don't want to make more of a pano and use BB's environment sphere, just use a box.  the thing is to give his IBL generator something to work with that works fairly well.  IBLs aren't very precise, so you don't really have to be precise with the environment.  also, we don't really know what we can't see, so you can have some fun.  just don't give the IBL generator a huge unnatural patch of a solid color.  once you have  a rough environment on all sides, load up his IBL generator and make some IBLs.  for anything lit, i use the method suggested by (iirc) Synthetic. and iterate my IBL renders to simulate the bounce.  but if everything is already lighting independent (like a background photo usually is), then i'd just do the one IBL render.