Another option (following on from Zac's suggestion) would be the generic laptop cases that (in Australia) almost all stores are now stocking. The laptop space could be used for cables, the file section for scripts/notes
etc and then there are several compartments of differing sizes that will accept adaptors/disks/pens/tapes etc. Most will also have a front pocket which expands so you can put fairly bulky items in there.
There has been a
shift recently to go from the standard zipper style, to the 'flap' style (like to postal satchels).
They have a standard carry handle and removable shoulder strap and many have eyelets on the zippers so that a small lock could be used.
I have had both but I now actually use one of the 'flap' style ones when I go to conferences and I always manage to pack a fair amount of things in it. I find that you can open it up more easily when standing up. The one that I have also has an additional divider in it. So there are three separate A4 (Letter) size pockets, plus the smaller compartments and pockets.
Another thing that I use for carrying those "just in case" items are the
clear plastic multi compartment boxes. I have two that always travel to shows with
me. One has all the different size and type of
fuse that my equipment uses. The other has batteries, connectors, adaptors, optocouplers and some money (just never know when you will need some and it is always when you left your wallet at home)