I have been looking for one my self. My current design is a 7 by 7 by 2 box with the spawner in the middle surrounded by hoppers. One wall is missing its center blocks so I can shoot them with my bow. I also use fire res potions so I dont have to worry about damage. Since I am in co op and I have 2 accounts, with a power 7 bow on the lvl 40 spawner that you get from the nether fortress island I can 4, sometimes 3 shot the blazes. I have an idea on how to maybe make a slightly more efficient one which I can smack with swords so I dont have to keep getting arrows, but it would take a lot of work to do and at this point it is the last quest I need to finish everything, so I am not sure if it is worth the time at this point.