mob grinder

im am building an adventure map with a mob grinder in it but first i need you're guyses help on how to make it so this is the first version of it and it is alittle off on the spawning cuz sometimes the zombie pigmen will spawn out of the structure and so i will fix that in 2.0 if you guys say yes about it.

so give diamonds for another adventure map and likes for the grinder :)

v2.0 -->http://www.planetminecraft.com/project/mob-grinder-2/

Additional Notes

don't for git to like and comment this project of mine so i now you guys are supporting me :)

v2.0 of this is in -->http://www.planetminecraft.com/project/mob-grinder-2/