Cheers for that Greg mate but its not that hard to do to be honest .. since showing you the mold right now is impossible i can at least tell you a very easy way for you to be able to make your own molds .
First off i use a piece of printer paper ( depending on the size of course of what your building ) ,,draw what ever you want to make on said piece of paper and use that as your template ,cut it out and lay it on a good size piece of plastic sheet ..i staple this to wood so it wont move..then just lay the template on there and lay your wood around it..pin it down ..a little jewelers hammer is brilliant here ,,instead of pushing the pins down and sticking them into your fingers ,,hit the buggers
Now the thickness of the wood all depends on how thick your build will be ,,i normally use 10x15mm balsa and 1x10mm base wood for bending,,if you want thicker just use thicker wood,,easy and cheap and reusable.
Once you have layed out the wood around the template mix up some extra thinned plaster of paris and pour it in and let it dry..the dryer the better.
A pencil is perfect for drawing on it once dry,,if you want real easy let the plaster day until its like chalk,then its super easy to draw on and then carve out your bricks .
When you have to build the supports for the bridge you can use Lego bricks to make a big block of plaster and work it once dry..easy as hell mate and once other thing if the bricks dont look perfect dont worry ,,just makes them look more realistic or battle damaged .,,easy
Phill