My wrapping paper box yielded a scrap large enough to cover the inside of the board. I just trimmed it to fit. (Here you see the back of the paper.) I have to tell you, the wrapping paper was even used. I just ironed it. Yes, I'm cheap. But hey, I didn't have to buy any!!!
Don't look too closely, it's not perfect! It would look a lot better if the frame was white or pink, but I'll settle for this for now. I'm really pleased with how it turned out for about $8. And it was such fun to make something with my girl.