there was an article in a RC car magazine about how exactly they do it. i lost the magazine, but suffice to say that the printing is most certainly done before pulling.
its pretty easy to calculate the distortion cos its all done by machiene, so it comes out the same all the time. the vac forming pulling machine used is pretty basic, but very consistent.
for your purposes, id sugest making a mini rig, with four rods mounted on the four corners of the perforated vac table which has the blank in the middle. this way when u pull down the lexan, it goes on straight and consistent all the time.
do you do R/C cars ? im just wondering.. if you would like to talk more, id be glad to share info with you, my email is reg_the_ruler@hotmail.com
hope this helps !!