Minecraft ps3 spawning problems?

I play minecraft on the ps3 and I'm making a minigames map but I'm having problems where I spawn. Every time I spawn I spawn in a different spot… So I made a 20x20 room around where I spawned the first time. I then kept on spawning on the roof. The roof is glowstone. I then put bedrock then glass on the roof. Although it helped spawning on the roof I spawned on the ground next to the room. I then took the roof off of the room but i spawned outside of the room again. Could my floor be the problem? The floor is iron blocks and glowstone blocks in a checkered pattern. I also true bedrock under the floor that didn't help. I want to know how to fix this because I don't want to be on survival and the other people spawn outside.