I don't think you should try making the dead rat 'till you have the mic in hand. I put mine together in 2 minutes with a sewing machine, wrapping a pice of fabric around the mic (with windscreen on) and sewing a seam across the top of the dead rat/mic and then continuing down the back.
My concern with estimating the size of the mic/windscreen and making the dead rat in advance is that you'll end up with a cover that's either slightly too small and thus won't fit, or slightly too big and could flap around causing noise.
Darrin