Based on a video (wSCutiy1hi4) by EthosLab on youtube.
I'm building it in the sky right now (multiplayer).
I plan to repeat the design stacked 5 times on top of each other for better spawn rates.
Each layer is 5 blocks thick horizontally and takes up 19x19 in the vertical dimensions:
Floor (also acts as roof for the previous level)
Water flows on this level
Spawning pad level is elevated from the water to prevent mobs from jumping back on them
And finally two air spaces...
...for the mobs to spawn.
Here is the floor plan. 1 pixel = 1 block:
MineCraftMobTrapFloorPlan.png (317 Bytes) Viewed 6104 times
Zoomed for your convenience:
MineCraftMobTrapFloorPlanZoom.png (1.8 KiB) Viewed 7898 times
Legend:
The outer wall:
Wall.png (146 Bytes) Viewed 6074 times
Water flows:
Water.png (226 Bytes) Viewed 6087 times
From the source:
Water Source.png (146 Bytes) Viewed 6079 times
To the hole:
Hole in the middle.png (144 Bytes) Viewed 6067 times
Each spawning pad is 3x3 with...
Spawning pad.png (202 Bytes) Viewed 6074 times
A half slab in the middle to prevent spiders from spawning:
I am suffering from a bug in SMP, which causes zero mobs to spawn due to a lightning glitch. (Basically the server thinks the light level is 15 when it's actually 0 all around. The sun somehow shines in through the blocks. Even at night. Scary.)
First, I am going to try thickening the walls where I originally intended to cut blocks to save on materials. (Mainly at the diagonal walls, I made them as thin as possible.) Then if that doesn't help I'll encase the entire spawning system in stone. (Quick and easy to do with water+lava.)
If both attempts fail, I will assume it happens because I built it at sky limit, at height 128(-3). Then I would have to tear it down and rebuild it closer to the ground. Worst case scenario: below height 64. Hopefully it doesn't come to that!