I don't have access to my save or demo for this room but I'm pretty sure that I know what must be done.
The most important point for this room is that you have to limit(not slow, since speed isn't an issue here) the briar's growth on the eastern side by dropping the trapdoors. The mechanic used is that:
Click here to view the secret text
×A square of briar with a pit next to it will not grow diagonally in any direction, not just around the pit. If you do this correctly in this room, you can make the east side grow two squares shorter by preventing it from growing around the corners.
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If you need to think outside of the box, then you didn't build a good enough box.