Extreme XP Maker

This spawner gets you tons of XP.

Get water, lava, flint and steel, mob spawning stone, and obsidian.

Make a 3 by 10 row of mob stone and hollow out the middle 1 block deep and fill with water, and at the last block deepen one more block and fill with lava. Then surround with obsidian.

Make portals around the structure.

Add 2 more obsidian layers around the structure and build up the last layer.

Put lava in between portals and wall.

build portals on top of each other as many times as you want (more layers better effect).

Build a portal in the middle.

On top of the portals build a stone layer in between. Then build 2 high wall of obsidian. After you put water so it flows inward.