How to make this file watertight?

I am having trouble getting this file water tight so that I can cut it on my laser. If someone has the time to help I would really appreciate it. The scroll patterns need to be open on the back side (shell) not filled in like when you print. If you cannot open this fie let me know your email address so that I can put it into Dropbox.

The file is located here in Dropbox:

do you have the original 3dm file you make when exporting it to stl? it is quite hard to manipulate irregular triangulated mesh.