TrainClown:
The secret building material was a tooth paste tube. You know, the old fashioned metal ones. They are made from a thin metal...
The metal is either tin or lead, it's heavy and soft. Lead is poisonous, but since we are not eating our models this won't be of great concern here!

(However take care with soldering, the fumes arising are not exactly healthy.)
Another source for this metal are the
metal caps which conserve the corks on wine bottles. Unfortunately today most of them are made out of plastic or aluminum foil, but if you find some older bottles they still might be made out of lead.
Where to get those bottles? Once I was lucky when I bought some wine in a winery. The owner showed me his cellar, and somewhere in the back I noted a box with two dozen of old, empty bottles. They had those leaden caps - or at least the rest of them around the bottlenecks. The wine dealer allowed me to cut them off, and so I could fill up my stock of lead foil. (I suppose he must have thought I'd gone nuts.

) Morale of the story: Try to get a look into old, dusty cellars of wine dealers, restaurants etc...
On the other side, at long last you have an excuse to buy that bottle of old Bordeaux wine you were longing for!

Be warned, however: This might turn out to be the most expensive piece of metal you ever bought for modeling! (But when you look at it this way, at least the wine is free!

)
Ron