Ah well here it is! the budget stand - thanks everyone for your tips and advice, here is the finished product.
Firstly it was really easy to do, was really cheap (only $2.41 per concrete block) and we recycled some slats from an old slat bed of ours.
We only used 4 blocks, didnt feel it needed a middle support as its only a 216litre tank, and could take 2 heavy peoples weight while bouncing in the middle lol.
The slats are 2.5cm in thickness, we double layered them so its 5cm thickness, plus a thin flat layer of wood on top to create an even surface.
We didnt used polystyrene, we used a blanket, Im confident about it, but will update you if the tank dies on us fingers x'd lol.
I think as far as cracking goes my tip on DIY tank stands so far is make sure the tank isnt hanging over any edges so that it is on a flat even surface. (DUH I know that sounds lame, but Ive seen people do it!!! have a tank that hangs over its base edges and end up cracking underneath :-? )
ANYHOO ENOUGH TALK HERE IS THE END RESULT