Help With Sides..

[ShadyB]

Ok, here's the problem I'm having..
On my main forum, I've made myself a custom design, fixed width and it looks great, with the shadows at the sides, much like on here..

However, I would like to know if this is possible with a fluid style?

Basically I want the small gray bits either side of my forum to work on my fluid style..

Here is my fixed, working style..
http://www.shadyb.co.uk/forums

And here is the fluid style I'm currently working on..
http://shadyb.co.uk/testvb/index.php?styleid=3

Any help will be greatly appreciated!
Thanks in advance!!