Creepers are by design extremely inefficient. Their spawners cost an absurd amount, and MOST skyblock players don't recommend them. THAT BEING SAID, Here's my design.
A7x7 room, with the floor as hoppers and signs on top of all the hoppers. On top of the signs is lava, so you can grind any level creeper but level 1 is recommended as this design kills most those faster than droppers.
Put the spawners on the same level as the hoppers and one block above that. If you have a 2 block thick roof you can put another spawner for a stack of 3.
(Note: This design should kill any creeper faster than a lethal drop suitable for the creeper)
You want this room completely encased, and I recommend glass. Have the hoppers lead to chests a few blocks away from the room, as cc can be weird. To prevent the occasional creeper outside the room, you can make the walls an extra block thick.
This is an extremely iron-expensive design, so have a stack of iron blocks and plenty of wood for chests on hand.
To extend this design, pick only one direction. Before you do any work, mine all spawners and pick up all lava. Extend the room one block in your direction, and put your new spawners next to the old ones in that direction. Reseal the room, replace the lava, and put the spawners down in that order.
This is my design. Hope that helps!