Eagle PCB is a great PCB CAD design software. There are no pre existing designs in CAD as this is usually intellectual property, and cost allot of time and money, if you carry on reading you will understand better. You may consider trying it your self and once you have a prototype you can get a consultant to fix er up for cheaper.
It takes some getting used to working between PCB layout and Schematics mode. These modes are used to validate basic circuits. There are allot of design aspects you need to know and usually gain as a professional. For a basic PCB like you need it doesn't really matter for prototyping but any commercial releases need professionals to implement design rules for mass production and FCC regulations.
First of you need to find out who is going to make the boards for you. I found Seeedstudio is really great and cheap! THey even do 4 Layers now, but that is used mostly for RF, 2 Layers will be more than enough. They can do various sizes, check the sizes you think you need, you can even split massive PCB and they will cut it to size for you.
Once you know the dimensions you start your CAD up and define the PCB borders. You find "SODIMM DDR2 sized (6.5cm by 3cm)" in the parts list and plop it down. From there, the compute module schematics becomes your new bible (If you only have 1 screen, print it out a few times). In CAD start drawing lines from the pins you need and slowly add new parts, like PINS, USB, etc, etc
Do not worry too much about how the PCB traces run, you will eventually get to a point where you need to lay everything out nicely, or in an ordered fashion. Concentrate more on the schematics part so it makes sense.
There are priority traces and less important but various rules apply.
- RF traces are highest priority, like USB communication, I2C channels, HDMI etc. These typically need to run uniform, side by side and far away from voltage or things that can generate noise. It would be good to use a ground plane too.
- Power always needs decoupling as close to the module or IC as possible. Even if the module has decoupling, putting another near the SO-DIMM and filter caps are pretty much essential at the input or near power regulators.
- Also it is good to make a main power trace that is thick, but runs far away from any sensitive components (RF Comms mostly, but on relays it doesn't matter) So if you planning to have 12V 2A input make sure it can handle up to 4A. (There is no precise way to calculate how much the trace can handle during prototyping, you just have to assume on the lower borders) You never know during prototyping what you need. Then run thinner traces to things that need power. A reverse polarity diode is a good idea and a fuse too but most power regulators have this built in.
- During prototyping breakout ALL the pins, even if they connect the VIA's and you don't expect to use them, you never know! You can always solder paths, add pins, or add stuff for testing and update the CAD design accordingly.
- Silkscreen as much as possible. Separate High voltage 220V/100V (mark it with a thick silk screen and text WARNING or something) and leave a good 2-4CM space between low voltage <12v circuits. This reduces the chance of noise and chance of voltage sparks or if something burns out it wont affect its neighbours. The minimum required is 1CM but that is highly frowned upon within professional designs, even simple power adapters.
The list goes on and on. Essentially you can use other designs to fuel project but always try and shop for parts before buying them, then place them on the PCB, as sometimes a part that can have an alternative may be very expensive and only needs a small design change to save allot of money.
Now after all that, have a CLOSE look at the IO board that is going to be released soon for the module. You will realise now, that things are placed deliberty in certain places and not just at random.