Tree fern fiber is similar to dried coconut.
Expanding foam is used to make backgrounds. Its stand up to water, is light weight, and you can stick dried coconut fiber to it with silicone.
If you read through this thread it will help you out a bit more.
About to build my First Vertical tanks! Update on page 2
Here is a link to an online store that sells cork bark. YOu can attach this stuff to the backglass of your tank with silicone, or you can attach it to the expanding foam.
http://www.blackjungle.com/Merchant2/merchant.mv?
This same website has a good explanation on how to make a background, and they use expanding foam also.
http://www.blackjungle.com/gallery/talltank/page1.html
There are basically two types of expanding foam that I know of that are used in the hobby. Great Stuff, which yellowish in color. And Handi Foam which is black/grey in color. You will get different opinions as to which is better. I have only used Handi Foam simply becuase I wanted my foam to be dark in color.
It would be possible to extend the background out of the tank. I suppose you would need to use a frame to support what ever it is you want to use as a background. I myself would just go out and a bigger tank, since I don't think it is worth it.
I think that light crates (egg crates) that they sell at Home Depot would help you build that frame.
Here is a link to the egg crates Im talking about
:
http://www.dendroboard.com/phpBB2/viewtopic.php?t=18135
Hope this helped!