I could see my main entrance from my second sentry-tree, which stood higher than the first one, but was in a less-advanteous position. The entrance looked rather conspicuous, so I started working on a way to take care of that. After some thought, I decided to "plant" a tree stump over it, and anchor my rope to that. This second tree was also in a good place to start dropping dirt from, now that the level in the bramble had also risen near to the top of it.
I dropped back down and clambered out the main entrance, and when in search of more wood, this time bringing my shovel and clippers with.
As the light started to fall that evening, I received a stroke of luck: a fallen tree lay in front of me, the trunk split and the base half-torn from the dirt. I separated to base of the trunk from the rest, and dug the roots free. Instead of heading back, with only my "front door" in tow, I kept working, spitting the tree further into planks, and by dawn I had more supports for the underground passage to my cabin, what should be plenty for the entire trip.