If my understanding of playing in the classical position is correct, then classical position basically means to have the guitar on your left leg. I also understand that the left leg should be raised in some way, so the guitar is on an angle and the 'butt end' is on your right leg.

*please correct me if I'm wrong.

So what should I be using to raise my left leg into this position? How high should it be raised to ensure the most comfort? I've been using a box that is about 6" high, and I've been noticing my lower back starts to get uncomfortable if I stay in this position for a long period of time (20+ minutes). My chest is perpendicular to my legs, my back is straight, I do not know what the problem is.

Any suggestions?
you just need to get used to it... I had same problems when playing my electric like that but later it was ok =)