Sram,
I think the best advice you could get on this topic, other than resorting to the Gallagher method of explosive demonstration, would be to abandon your idea of creating a mini-nuke. Creating a controlled and small nuclear fission or fusion reaction is a topic well out of the realm of this board and probably out of the realm of anybody not possessing some sort of security clearance. You could easily go to the library and with no small effort master the theory behind nuclear reactions and weapons but thankfully, the practical knowledge and experience needed to actually construct such a device is not easily obtained. Even if you somehow got your hands on some nuclear material and then somehow managed not to irradiate yourself during the construction, I would expect your door to be broken down by some angry men in suits and dark sunglasses well before you were ready for your presentation. I understand that you want to demonstrate the destructive power of nuclear weapons and give the audience some basis for comparison but I'm sorry, that's probably just not possible without actually seeing one go off. There is a lot of stock footage floating around of nuclear tests performed during the 50s and 60s that are pretty scary to watch. I think some of them even involved live animals like pigs and dogs, etc. I'd spend my effort looking for those. A picture is worth a thousand words and if you blow up your audience, they can't appreciate the gravity of your message.