Tranmaps in Boom, how?

[DU0]

Hi. I was reading up on this thread earlier today, where the author says the following:

 

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Now import it into your wad with whatever lump name you want, and use the Save As Doom Flat menu item in XWE to put it into the correct format.

To use it in a map, tag all the lines you want to make translucent, but do not set special 260 (translucent line) on them. Put a line in a dummy sector somewhere that has special 260 and that sector tag, with your tranmap lump's name as its middle texture. If you've done it right and you're using a Boom-compatible port that's not ZDoom, it should work.

 

Well, everything was working up until this part. When I try to set it as the middle texture, it won't show up because it's a flat, and if I just try to set the name of the flat as the middle texture, PrBoom+ crashes on startup. How can I make this work?