What about just making the entrances to the kolkano defense-secure, and make some sort of fence around it, so any mobs would be kontamikated? Or make the lava rivers run down into a lava moat?
A few controlled lava rivers on the outside could branch on the way down and provide some light on the ground, and make it look like a real kolkano.
I think obsidian would look awesome, but it'd take a long time to get.. *Falls asleep*