I made a checkered flag cake once (here's the link, sorry, I do not know how to post the actual photo into the body of my reply: http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1181256
) and the inside of the cake actually is checkered like the icing pattern on the outside. Since I can never figure out the easy way to do things, and actually always take the complicated route...this is what I did: I took a cake board and cut it into strips that were the length and width of my cake pan. Then I cut slits into the strips of cardboard to enable them to slide together. (In a perpendicular form) I ended up with three 13" strips and four 9" strips (It was a 9"X13" cake) that, when assembled, formed a grid pattern. I then set the "grid" into my greased cake pan, and proceeded to fill each square section formed by the grid with cake batter, in alternating black and white colors. When I filled up the first pan, I just lifted out the cardboard "grid" and placed it into the second 9"X13" pan and repeated the process. The cake came out looking just like a checkered flag. In fact, I used the pattern on the cake as a guide as to where to place the icing.
I do hope this helps you in some way. Good luck!