If you need it right away, you can always use a cereal box...that's how they used to make patterns before we were spoiled with acrylics. Bev, I hate to measure...and I do mean HATE....if you have a nice rotary cutting mat, you can use your larger ruler in conjunction with the markings on the mat to get your cuts easier. I usually have the hubby cut them out of foam core which works perfectly, can be re-used hundreds of times and we get from his work. A nice piece from your local office supply store would't cost much, you can make several shapes from one and it only requires an exacto blade to do.
*if you use this method, score both sides of the poster board before cutting through the foam center*