Howdy Mao! I will preface this by saying that I'm pretty new to this stuff! I'm in the process of putting a built in tank in my house (currently under construction). I've been looking into doing the same thing you are, with regards to a custom background, but have come across something you might want to look into. Instead of constructing the background from natural stone I have been looking into using styrofoam with a coat of either fibreglass or quickcrete. They look amazing and, if done correctly, very natural. Below are some links that I found very useful in my research. Hopefully they will help you out if you decide to go down the Styrofoam path.Good luck!
Courtesy of "the deeb": http://www.monsterfishkeepers.com/fo...&highlight=65g
and: My 300g paludarium journal (56k warning!)
They are both from the same gentleman, but he is extremely thorough in his process, I hope this helps!