Development Assistance Nether/end custom chunk help

so this is an odd thing i need help with, i run a skyblock server and we are implementing a nether and end that reset on a predetermined basis. But in testing we almost always seem to spawn underground or in lava or over void. I need help with a plugin or way to clear a certian space around a set spawnpoint no matter the world(which is randomly generated) and have a layer of blocks under the player. So say i want the nether to spawn players at 0,75,0 but it was a solid area i want it to be cleared in a 16,16,16 and a layer placed at the bottom to prevent falling, alternately if they end up over void or lava it places a floating platform so they dont die.

Sorry for the wall of text.