There is no denying that the traditional playing position for classical guitar using a footstool is not very ergonomic. I could never play for very long without getting a stiff back and/or pins and needles in one of my legs. And so I began looking around for alternatives.
Basically, these fall into two categories: devices that are attached to the guitar, usually by suction pads, and those that aren't. This cushion is in the latter group. I did not like the idea of suction pads on my guitar, which I thought might create unsightly marks, and I doubted how securely they would hold the guitar in position. Therefore, I chose the Dynarette cushion.
I have been very happy with my choice. The cushion is well made and does the job. The bottom surface is made of a non-slip fabric that keeps it in place on the thigh. It is simple and effective. The only downside I can see is that it would take up quite a lot of space in your case if you were touring, but this has not been an issue for me.