I have been doing a challenge recently where I play on a flat world (only glass, abandoned mineshafts, strongholds, dungeons, lakes of water and lava)
But the problem I have run into is torches, I can't get any coal anywhere (neither charcoal)
The only source of it is in the nether(which is also flat) from wither skeletons.
I would like to make a no redstone mob grinder in my nether, I have some Ideas, including using rings going out inbetween the 24 and 32 limit for moving mobs along with it going up into the sky, using signs to make the mobs drop off the edges and lava streams to push the mobs to a central location. (along with slabs covering almost all the ground I don't want them to spawn in)

I was just wondering if there was a more efficient way to do it (both mob spawn count and material wise)

If ya'll have any suggestions, they would be greatly appreciated (perhaps even pictures or videos)