If you are allowed to use thermocol, you may follow these steps
firstly, take an appropriate size of thermocol (as big as you want it to be)
Secondly, leave the thermocol as it is.take another thermocol and cut it into rectangular pieces with a knife burnt in flame of candle -- advisable for smooth cutting.
now make such rectangular pieces of sizes -- they should be in order like one is big and the other one smaller than the before one and the other even more smaller.
now stick these pices one above the other..
if you understand this.. im sure not
pls contact for further doubts and information and refer to internet for clear process.