The big screen itself is quite easy, but SLADE has problems with it. When you convert an image to the planar format (you need to have additional image formats checked in SLADE's settings), you mustn't click save to confirm the changes but rather click on Revert and confirm that. If you click on Save, you'll get a strange PNG file that either works in most places (not in the loading screen) or it gets strangely corrupted and broken. If you click on Revert, it will work. Enjay actually
figured it out in an old thread.
The notch graphics were a bit worse, because they're supposed to use STARTUP's palette, but SLADE can't use it, so the best solution is to make it with a hex editor, since it's all raw data (one hexa character is one pixel in the graphic, so it's easy to follow). That beam was easy to make, but something more complex, like the original bone notches, would be tougher and borderline impossible if you wanted them to use different colours than those in the main image.
To be honest, I'm surprised there's a support for something as niche as this, were planar graphics even used at this point in 1995 other than the loading screen in Hexen?