Marc,
I took pictures of my travel case tonight. I'll post one here and you can go to this link to see the rest of them...
Travel cases
The box is a standard size for holding US size hanging folders. I would assume that there is something similar available in metric sizes where you are (Europe right?).
I told you that I use a bolt to mount the figure to the base. This will work just fine but I actually do it a little bit differently. What I use is a piece of hadrware that comes from a trophy. I was able to get a handfull of these and they worked nicely so I used them. It's a cylinder that's threaded on the inside and has a hex base to it. I put this on the top of the base plate and attach it with a bolt from the underside. You'll be able to see what I'm talking about in one of the pictures.
Also, I added some pictures of another way I can transport a single figure.
It's a clear acrylic box that I bought at a local crafts store. It's also very inexpensive. One end comes off of it like a lid. I took this end and drilled a hole through the middle and added a bolt and nut just as I did with the big box. It's also a nice way to display a figure and keep the dust off of it. Again, you'll see what I'm talking about in the pictures.
You had also asked for some dimensions in metric. The 5/16" bolt is about the same diameter as an M8 bolt. As for the length, it only needed to stick up above the nut by about 20mm (3/4"). So with the plywood base at about 6mm (1/4") thick plus the nut plus the 20mm extension, a bolt maybe 35-40mm (1 1/4"-1 1/2") long should do okay. Then just drill a hole maybe 1/2mm (1/64") larger in diameter than the bolt and long enough to fit most or all of the bolt extension into it. The more bolt in the hole, the more stable the figure will be.
For the boxes and the base plate, I put a 30cm long scale next to them so you can judge about how big they are.
I hope this helps you out. If you have any questions, please feel free to ask and I'll try to clarify anything I can for you.
Good luck!
Craig