If you spend as much time as I did scouring the web for every bit of information available you'll probably get as confused as I did at first.
There's a lot of good info out there but there's a lot of rubbish too.
As a rule of thumb the greater the number of people say something, the more likely it is to be correct. Usually. But not always.
There are already some "Instructables" on panel building on this and other sites that contain credible and very useful information.
Very few of the articles etc. that I read, however, pointed out the life expectancy of the Solar Cells you'll be using versus the materials used in constructing the DIY panels.
The first Solar cells ever made still produce electricity and look likely to continue doing so for many years to come!
A piece of painted plywood used to mount your Cells on, on the other hand, will have a very limited lifespan under the hot conditions inside a panel.
Most articles I read are concerned with building panels "on the cheap" as I did.
I built more than 300 watts of panels, all on plywood.
I will eventually need to somehow remove the Cells from the plywood and rebuild the panels on something more substantial, not sure what ( or how ) yet....maybe aluminium or Tedlar? Not a job I'm looking forward to but all part of a fairly steep learning curve.
Think outside the square and make Google your friend. A search of "solar panel backing" returned 98000 results including this one.. http://www.treehugger.com/files/2008/08/cotton-castor-beans-combined-solar-panel-bio-backsheet.php
My advice......research thoroughly then do it once and do it right.