LOL, With sps I think it is a bit risky leh... A slight change in pH will affect the sps??
I just align the small rocks on the eggcrate.
Then do my cement mix with the sand and coral chip.
Next fill in all the gaps and where the rocks meet so that they will stick together.
Den leave it to cure for 2 weeks. 1 week in FW and 1 week in SW after it harden
As for the placing it, I just lean it against the back glass and get a heavy rock in front of it to keep it from toppling... Den okay liao.. But you need to measure accurately the space that you want to place the backdrop...
Will show a photo if I can...