Put it together:

Video: Assembly of a cloudlet case (by C4 Labs)
https://www.youtube.com/watch?v=ZUDcgoOgY_A

Give your Pi's hostnames:

I have 6-8 shells  open while going through this exercise. 
update all of your PI's hostname files.
I have a 6 node cluster so my pi's each get a bit 000000 identifying it's place within the case.

sudo nano /etc/hostname

Add the server hostnames to the file

ubuntu-pi-100000
ubuntu-pi-010000
ubuntu-pi-001000
ubuntu-pi-000100
ubuntu-pi-000010
ubuntu-pi-000001

Give your Pi's IP's:

Update the netplan configuration on each of your Pi's

sudo nano /etc/netplan/50-cloud-init.yaml

Modify the network file to match your adapter and IP's.

network:
  ethernets:
    eth0:
        addresses:
        - /24
        dhcp4: false
        gateway4: 192.168.1.1
        nameservers:
            addresses:
            - 192.168.1.1
            search: []
  version: 2

Reload your network configuration

sudo netplan apply