I copied my basic design build from my 130k Trap.

It took about 4 days (6-8 hours each day of gameplay) to build this trap using items I've mined. The hardest part is getting the materials for the pistons and repeaters, i.e. redstone & iron. If you can make all of your pistons, then you're pretty much set. You might think getting slimeballs is the hardest part, but once you find a chunk or more that spawn slimes under layer 16, it's easy to just grind them away as they spawn to get the amount of slimeballs you need. (For the basic design, 40x40x40, atleast).

Using the cleanroom & MCEdit, I made a floor list that way if anyone would like to add floors to the trap they'll know what they'll get:

(03 Floors) ~ 06,000 I/H - This is the original 40x40x40 design
(05 Floors) ~ 09,000 I/H
(10 Floors) ~ 19,000 I/H
(15 Floors) ~ 29,000 I/H
(20 Floors) ~ 34,500 I/H
(25 Floors) ~ 44,000 I/H
(29 Floors) ~ 47,000 I/H - Bedrock layers -> sky limit, presuming you can stand somewhere at the lowest part of the Bedrock.

I built this to see if it would actually be desired by myself for later use in new worlds, and I wanted to see how long it would take. I planned to make a new world when 1.8/1.9 changes gameplay a bit and start fresh, and I needed a new trap I could make without weeks of effort.

Additional Notes

With the 40x40x40 design, you get around 100 items a minute, or 6000 items an hour. That's not too bad for the dimension of the trap. In multiplayer, this should be a cakewalk with good teamwork. I took it and copied it to a cleamroom map, to see what it can do at 100%. Basically, you just have to light up all caves and ground in a 144x144 area which won't be very hard if you know how to mark off your spaces, and you need to get materials for building anyways. When I first make a world I spend 3-4 days just getting minerals from caves, so by the time I'm done most of the space I need lit is already torch-raped. With the amount of redstone & iron you need to make pistons, you'll be in caves for awhile anyways. After that, it's just going around and hunting mobs down with oldschool listening since the F3 key doesn't do what it used to. However, if you look at the E:xx It will tell you how many entities are on screen, so it's possible those may be living things. Even without lighting up much caves, if you afk for an hour or so you can still come back to items nonetheless using this trap.

The best part about it, is if you like you can always add more floors later on for more spawning space. For example, add a single floor each day in real life, and you can have this thing up and running a possible max of ~ 47,000 items an hour in less than a month after the original 40x40x40 trap is built. Multiplayer, I have no idea how fast it could be made. The main rule to remember is the more space mobs have to spawn, the faster the game will let them, thus increasing the drop rates.