Get unique device id for every amazon echo devices

I need to make a Custom Skill for the Alexa Skills Kit, which should be one echo device in every room. I need to get the device_id for every echo device. I heard this isn't possible, but maybe it changed, or if not is there any other way around.

How do I find my Amazon Alexa device ID?

Amazon Echo The wireless MAC address for your Echo can be found at the bottom of the Echo Setup screen in the Alexa app.

Can you have multiple Alexa devices on one account?

You can have as many Alexa devices on one account as you want. Amazon has not set any limits on how many Echo speakers can be used by a single account holder yet. If you own more than one Echo speaker, you can connect all of them to your Amazon account.

You can do it now! context:System:device:deviceId

As far as I can tell it only works on real devices. So if you are testing in the developer's Skills Manager you don't get the field, but when used with a real Alexa device, it works.