Well I managed to find a box 30x24x20", which was close enough, for £8.95. I had to cut the height down to fit within the Parcelforce limits of girth+length<=3m (just got in at 2.98m and just under the 15kg limit at 14.8kg as well) otherwise I would have had to pay an extra £20 or so for the large parcel service.
Happily, using xan_user's suggestion to use a cardboard panel screwed to the fan and taped down to the case seems to have kept the PC intact and it booted up fine. I couldn't get my brother's old PC to see the new one on the Network (or vice-versa) for some reason, so I installed Filezilla server and copied across his data files that way (which we had to leave going overnight as there was about 190GB to transfer and it was going at about 5MB/s!).
It did lock up and so I lost the Teamviewer connection when I tried running HwInfo64, which it had been doing with the Nebula TV card installed but I thought I'd resolved that. Anyway, it booted OK later and I showed my brother around his new PC and then we installed 7 Windows Updates and I think Avast updated as well. Unfortunately, in the process of unplugging/replugging his USB printer cable in an attempt to get Windows to install the drivers, he managed to knock the power cable enough to cut the power! I have no idea how he managed this as I never found the connection to be dodgy and I'd already told him to make sure it was pushed in securely as when he first powered it up he said it was a bit loose and crackling.
Anyway, when we rebooted, it came up with the "Windows did not shutdown properly. Start Normally or Repair" (no safe mode, last known good config, etc for some reason) and it just BSOD whenever he tries Start Normally. It's probably one of the Windows Updates rather than the power cut that's buggered things up but it's rather depressing after spending so long tweaking the damn thing to perfection that this has happened. I've got a EaseUS ToDo Backup on there which we could try restoring to but that's dated 30-08-2013 and I probably adjusted some thing between then and packing the PC up to ship, so I'm going to see if it's fixable first. It BSOD even booting into Safe Mode with networking at the moment (I had to create an additional Windows boot menu entry "Win 7 (Safe) with BootICE and set it to use Safe Mode, because as I say the default entry strangely doesn't offer Safe Mode, even if he presses F8) and only Safe Mode (without networking) boots but of course I can't then access it via Teamviewer to fix. I've had him remove the Nebula card and even tried a clean boot and disabled the onboard Qualcomm Atheros NIC but it still BSOD.
I've got a Win7 x32 VHD (Portable) on there which is working and we managed to install TeamViewer Portable in that and so I can edit the offline registry for the Win7 x64 install that way, so hopefully I'll find a way to get it working again eventually