First, Welcome to MAAST!

You can sometimes find flat pieces at the LFS. Might call around and see which stores in S.A. have some.

As far as securing and shaping, one method is to use clear acrylic rod to hold the pieces together. I use a masonry bit to make the holes and set the rock up in a tub outside the tank to get it like I want it before placing it in the tank. HTH.