Worked from memory. The stair portal isn't totally perfect, but whatever.
(Some textures come from the OTEX.wad texture pack)
@tuxic Wait, people already went the extra mile to do it : https://www.youtube.com/watch?v=R9_YI_ex3Cs
the mastodon instance at cybre.space is retired
see the end-of-life plan for details: https://cybre.space/~chr/cybre-space-eol
And as soon as I post the video, I got a solution for a seamless stair effect, go figure.