I was shopping for some velcro and other things I was decided to build my carrying solution with, but I found something I thought might work quite well. And now when I've tried it, it does.
It's a beltcase for camera. The Beagleboard with the case fits there well, even with USB hub and every other thing I need. That smaller pocket is for batteries, they should fit well too. I'll just make a tiny hole there for the USB cable and cut one of those, because now it doesn't need to be retractable.
I have ordered a battery case for 4 aa batteries for testing purposes. People have succeeded running the board without frying it, with just 4 AA NiMH cells, so it should not be a problem. (I'm not an expert and I'm not responsible if you burn your board with those batteries!) Now I have only 2700 mah ones, but I'll get some better F-sized 16000 mah cells which should be enough for me.
I'm extremely pleased with this project. Now all I need to worry is the keyboard, which still doesn't work, mostly because I haven't given it any attention for a while.
It does look quite nice doesn't it? Worst thing is that display plug sticking outside of the bag. It's a bit bulky too, but it's definitely a lot better than a backback or a messenger bag.
I've also cut that display cord again, to make it a lot shorter.