Which Lowes are you going to?
The piece on top of the fan/filter in my current avatar (that's my clone - Ryan - who lives up to expectations that come with that far too often

) is a stock piece from Lowes in Cary. I mean bar code and everything; I pulled it out of a stack. There were several different sizes and thicknesses available. I think I paid around $5.
Edit - in case I change my avatar, here is the one I was referring to:
