I am in the process of making a background for my 90gal (5' long x 20" wide x 18" high) tank.
The background is currently at the stage of siliconing into the tank and rendering the joins. Though some may like to see it progress
Basic foam work - 30th Dec 2013
Expanding foam added - 30th Dec 2013
Tank fitting - 3rd Jan 2014
1st layer of render - 11th Jan 2014
2nd layer of render - 11th Jan 2014
3rd layer of render - 13th Jan 2014
Some fake rocks made - 14th Jan 2014 (these may or may not be used in this tank yet)
Background detailed and ready to be put into the tank - 14th Jan 2014
So yep, this is where i am up to. Today the background will get a bit of sealer around the edges, then let dry before siliconing in. Once in the tank the joins will get rendered and detailed, then it will be left for 2 weeks to fully sure.