I too could not bear to watch the Hawks (though honestly reading this thread is a bit headache inducing as well!)
In the back of the seat, on top, there is an opening that is attached by velcro. You open the velcro and slide the cardboard in. I actually cut the cardboard to make it a perfect fit (it is a bit of a trapezoid), because I'm a little a-retentive like that, but I'm sure you could just cut a smaller rectangle out. Anyway, you just slide the cardboard in, reattach the velcro, and VOILA! Otherwise, you can order a PE board from baby jogger. It is $8 plus $7 shipping.
HTH!