As requested, here's some info:
First off, here my tank and the DIY background:
What was used:
-3 Hard Styrofoam Blocks of 2’ high, 2’ wide and 8 ”thick
(Any other thickness can be used but thinner will resolve in less structural possibilities.
-Couple of tubes of Silicone (with application gun)
-A kilo of 2- component Epoxiresin ( this of course depends on the size of the background.
-Several different sized brushes
-Sharp kitchen knife and Stanley knife
-A gas torch or big turbo lighter.
-A big pile of patience!
How it was done:
The background was build up by using hard styrofoam blocks.
The blocks were cut into the rough shape of rocks using a sharp knife.
All the tiny bits of styrofoam will make an incredible mess and a hairdryer can be used to blow the bits and pieces of the artwork before you continue.
After that a torch was used to melt the edges of the cut off parts, to get them rounder and in a natural looking rocky shape.
The major part of the frontside of the blocks was epoxied outside of the tank.
The backside of the blocks was cut into shape, so that the outer 2 blocks would fit exactly around my 2 triangle shaped dry-chambers( one as overflow, the other as output for my filter and wires).
The side front of the background overlaps the side of the overflow with half an inch from top to bottom.
All 3 blocks were fit into place and secured with a lot of silicone.
After the silicone was applied, wait 24 hours for the silicone to dry.
After that the seperate blocks were epoxied together and finished of the upper edges with epoxi too, fitting and attaching the whole thing to the upper glass ledge.
In the overflow edge a comb of plexiglass was attached and epoxied.(Broken now, this should be done better.)
The background is cemented in the tank by the epoxi and cannot ever be taken out anymore.
This wouldt not apply if you can put the background into place in one piece.
That wouldt be a lot easier, for the whole background would be epoxied before siliconing it into place.
It could not be put into my tank in one piece, because of the glass front to back braces on the tank.
Brown coloured epoxi was used so there was no need for a layer of paint first, but the best option is to paint the Styrofoam with some layers first, let that dry out and then use clear epoxiresin.
The glue used for kitchen tiles can be used as well instead of paint.
That looks nice too, but several layers should be applied then, with 24 Hours of drying in between the glue-paint-jobs.
After the layers have dried, take some fine sandpaper and rough it all up a bit.
Any brushmarks can be sandpapered away.
Epoxiresin can be bought at any boat repairshop and isn't that expensive.
It is a lot of work though, for the epoxiresin will dry out very quickly.
This resolves in dryed out brushes and for my background around 12 brushes were used.
Adviced is not to make any more epoxi at a time then one can use up in 15 minutes.
You can apply some sand in ditches and corners to create a somewhat more natural effect, for the epoxi itself will be a bit shiny if it dries.
The epoxiresin should be used with a harder that you can buy at the same boat repair shop and mixed at a 3:1 rate. Most shop will sell 2-component epoxiresin in one box.
During the use of the epoxi the fumes that come off are toxic and hazardous to your health, but when it's dry, it's totally harmless for your fish.
So allways do the job in a well ventilated room, and using a face mask will certainly help .
Although i suspect some members might like the effect of those fumes.
This bit is personal and opinions may differ:
After 24 hours the resin is dry but the tank should be left to cure for three weeks.
Than the tank can be filled and a pump should be attached for the movement of the water.
The pumps should NOT be a filter you use for Tank filtration, for there still may be hazardous toxins in the water . A simple pond pump will do.
Daily 50% waterchanges should be done in that week to wash out any remains of toxins.
Some say after a week of 50% wc that tank should be fish ready, but I advice to let it cure 3 weeks prior to the washing ritual.
After that month the background will be extremely hard and epoxi is the best background component that is tough enough for prolongued Pleco raspings.
Given some time the background will become natural looking by some algae that will buildt up .
All in all, if you got the time and got some ideas of what you want and like, using epoxiresin can help you create a very personal background at limited cost.
The hardest part may be the wait for the month of curing of the resin to pass,
before you can finally start the rest of the aquascaping.
And of course putting the fish in there!
thanks for giving us the idea and info on how to, even if some of us are now left puzzled and unsure how. a very good post none the less . 
:angry: 
