I really love reward chests ^-^ and I got some passive mob spawners from them, and it took me a bit before I found a design that decently works, for a No kill system. It wouldn't be to hard to make it a punch kill system ether, just leave out the lava! So I wanted to share it. Remember this is how I set mine up, I'm always down for suggestions if there is a better way to do it 
So basically with the 1 block high mobs, that need grass to spawn on, you can have a row of grass, the one block behind and up from the grass a row of pistons, your choice, then behind the pistons, [the side not wood] a row of blocks and a line of redstone on top of those blocks.
Then you can feel free to attack the line of redstone to a clock, I chose this small one, that is hidden in my wall, and I built on top for show The clock is a repeater on 4 ticks feeding into a block with below has a torch on the block that the repeater is under and on the other side of the repeater a block and under that block a piece of redstone.
Now the have to be pretty close to the grass within 4 blocks (I believe) from the spawner, so you have to make sure to place your spawners close. So in this case, I put the spawners above the whole where they drop down, and 1 block above the pistons. (That way the spawner doesn't get jerked around!)
Then if you want to cook it, It's not 100% but, I put lava on the one block the water doesn't reach, (if you set up your water that way). Not all of it is going to get cooked because some of the animals will die in the water
Now with any animal that is 2 blocks high, cows or sheep, you can use a designs where when you put down the line of grass you put either a stone or wood pressure plate on top of the grass, the behind that on a row of blocks and on the row of blocks, (the same highest as the pressure plates) put a row of repeaters facing into a block and on top of that row of blocks place sticky pistons and on the sticky part, but the blocks on it then when the animals spawn will be pushed off the grass into the system.
Hope that helps! I'd be happy to have feedback.
So basically with the 1 block high mobs, that need grass to spawn on, you can have a row of grass, the one block behind and up from the grass a row of pistons, your choice, then behind the pistons, [the side not wood] a row of blocks and a line of redstone on top of those blocks.
Then you can feel free to attack the line of redstone to a clock, I chose this small one, that is hidden in my wall, and I built on top for show
The clock is a repeater on 4 ticks feeding into a block with below has a torch on the block that the repeater is under and on the other side of the repeater a block and under that block a piece of redstone.
Now the have to be pretty close to the grass within 4 blocks (I believe) from the spawner, so you have to make sure to place your spawners close. So in this case, I put the spawners above the whole where they drop down, and 1 block above the pistons. (That way the spawner doesn't get jerked around!)
Then if you want to cook it, It's not 100% but, I put lava on the one block the water doesn't reach, (if you set up your water that way). Not all of it is going to get cooked because some of the animals will die in the water
Now with any animal that is 2 blocks high, cows or sheep, you can use a designs where when you put down the line of grass you put either a stone or wood pressure plate on top of the grass, the behind that on a row of blocks and on the row of blocks, (the same highest as the pressure plates) put a row of repeaters facing into a block and on top of that row of blocks place sticky pistons and on the sticky part, but the blocks on it then when the animals spawn will be pushed off the grass into the system.
Hope that helps! I'd be happy to have feedback.
