The Bounding Box Outline mod will show you nicely-shaded bounding boxes for all Structure Pieces placed in the world, including Nether Fortresses. It's currently available for 1.6.x and requires Minecraft Forge.
I've already got a Forge profile laying around, so it was a simple matter of dropping the mod in a fresh game directory along with the Skyblock 2.1 save, an edit to the level.dat
to enable Creative, and lo and behold:
So yes, there are two Nether Fortress "ghosts" not far from the glowstone island.
The nearest corner of the one visible on the left is at approximately 76, 62, -63 (XYZ) and stretches to the North and a bit to the East from there, not including a hallway protruding to the West (the direction back toward the portal, but farther North than this corner).
The nearest corner of the one visible on the right is at approximately 94, 71, 73 and stretches East from there with a Southward and downward tendency.
You could go off those numbers and just grope around trying to make a blaze-spawnable area without the mod. If you want to use the mod though, it's quite easy to install starting from zero modding experience: download and use the automatic Forge installer which will create a new forge-enabled Profile, then drop the Bounding Box Outline mod's zipfile into the new .minecraft/mods/
folder that will be created after you run Forge once. Once in the game, turn the mod's visuals on by pressing F3+n (simultaneously) and the fortresses will show up bright as day.
Assuming you're on creative with cheats enabled and on version 1.7, you should be able to place down a command block and insert this command into it:
/setblock ~ ~ ~ mob_spawner 0 replace {EntityId:Creeper,SpawnData:{powered:1}}
Then power with any redstone device, and the command block should turn into a charged creeper spawner.
Best Answer
Jungles in Minecraft were made much rarer in the 1.7 update, so if you are using that they're going to be much harder to find. However, biomes will now place themselves next to other biomes of the same climate, and because Jungles are warm, you should look near Deserts, Mesas, and Savanas. Temples inside these jungles are difficult to find, and not all jungles have them. It can help to burn down some of the trees, since because the temple is made of stone, it will be safe from burning. Aside from using mods or seeds, thats really the best you can do.