Well, here's my bright idea for that and now I have cake boards the size I need AND they are re-washable/re-usable. Not like cardboard you have to discard after each use.
I purchased these disposable cutting boards, about 5 or 6 in a package, at Bed Bath and Beyond. Here I'm cutting a six inch circle to use on a six inch cake as I do the upside down frosting technique. I trace out the circle using food safe makers and cut out carefully with an exacto knife.
The photo to your right, I'm holding the top as I take off extra frosting. This cake hadn't been frozen yet so I was really gentle taking off that frosting so as not to tear my soft cake. So not to self: always, lightly crumb coat your cake, freeze and then do this technique (they're just so much easier to handle :-).
I also used the circle to mark the center of my cake board so I'll know where to place my cake.
Then I used the same board to cut out a circle of fondant to cover the top portion of my cake and......
Here ya go :-).