By big rocks I means rocks at least as big as the concrete blocks you use on your lower retaining wall. I have found out that over time when I used anything smaller, that they were just too easily pushed around by soil movement, especially for retaining wall purpose.
If you went with two boxes instead of one on your tunnel for access, it would probably make track installation and maintainence much easier. If you have the one box already installed, don't worry about it you can probably get by. I would just use dirt or crusher fines to make the floor of the tunnel, because it is cheap and easy to move around. Hope this helps.
Look forward to seeing your progress.
Terl