"I have a great mental effect there I can produce a geometric shape on a wooden board, so i need to get their thougts from some kind of device before I produce the shape on that wooden board!"
Well, you could start with the classic "think of a geometric shape inside of a geometrick shape" where you will usually have your particpants think of a circle with a triangle inside it or in the reverse.
If you can do this with any shape and require a "peek" you have a large variety of choices. Guy Bavli sells a clear clipboard which you can peek the shape from after the participant has drawn on their paper and retained the paper.
There is also the Buma board which is a similar device to Lee Earle's Clone Pad which allows for a real time peek of the design drawn. There is Son of Mind Scan which are pads of paper that will give you a real time peek (I love them) but they are a finite resource and require refills. The other two are not.
You could use a peek wallet such as the mind spy wallet, a Jaks type wallet (my preference) or even a basic Himber Wallet can be used for that. (See Mind, Myth and Magic for a description of that). You could also use a simple centre tear or other tear type effect. I prefer one like Martin Gardner's, Barry Richardson's or Dick Osterlind's (See The Purloined Thought for those references) or try Peek Performances for some excellent ideas using simple cards and/or envelopes for no tear methods.
Hopefully that will give you some ideas, if you want more and there are, let me know.
Paul Alberstat http://www.mindguy.com