You could just use tools/toolsclip since it's intended to affect players only (PLAYERCLIP for players only, CLIP for players & NPCs (Halloween Bosses in TF2's case)).
I was originally using that. However because my clipping becomes steeper than 45° around that area you see in the video, you can't walk up it. The "stairs" tool texture lets slopes be as steep as 63.4349488° and stay walkable however.
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