Thanks Pito - took me like 3 hours to write that
Honestly, you can make these as thick or thin as you want. If you use Rhody's idea of fortifier, he said you could get it as thin 1/4". The thicker the pieces, the stronger the pieces will be. My panels are typically 3/4" -2 1/2" deep, and that varies along the panel.
I don't like to use anything other than regular cement, so I just use a slightly wetter version of my Preferred Recipe, minus salt - replacing salt with sand and shell. I will sprinkle salt on the top to add porosity though.
I still am unsure as to why the acrylic is needed, unless it would be for easy removal. If the tank is unfilled (as you say), and you cast your pieces as I and Randy suggested, straight onto glass (protected with plastic - it will stick somewhat to glass), the back will be smooth and once you spread the silicone on and attach them, they will be in place until you remove them - not much in the way of space to trap anything.
But if you really must glue these to acrylic, you might just save yourself some trial and error, and just jump straight to the two minute epoxy - that Will hold it to acrylic
If your tank is wide from front to back, you could really do some neat things with this - it can really add an illusion of depth if done right.
Keep us posted!