I installed my 3D background to my already up and running 180 with very few problems.
First I moved the gravel away from the back area of the tank and then I carefully lowered the panel into the tank and maneuvered it behind my overflow box and flush against the back wall....once that is done I pushed the gravel up against the bottom edge of the panel and secured the top edge with plastic clips every few inches.
It may not be as perfect as siliconing the panel to a dry and empty tank but it has worked well so far.