I assume the "clean flat board" is for building the bridge support structure?
Correct. I plan for it to have a central rectangular hole 30 x 20 cm for the bridge to span. The board itself will be supported between tables/benches.
I've been looking at some ideas for weights for a project in my home shop. If I come up with something soon, we could certainly use those, too. If not, I might (_might_) be able to borrow some from work. We have a supply of 2.5-pound weights from Sports Authority that we use for our in-house testing.
We have a fairly big pile of aluminum ingots that we can probably select from and modify to get close to the right weight.