Using wildcards in ssh configuration to create per-client setups

In my role as a linux consultant, I tend to work with a number of different companies. Obviously they all use ssh for remote access, and many require going through a gateway/bastion server first in order to access the rest of the network. I want to treat these clients as separate and secure as possible so I’ll always create a new SSH key for each client. Most clients would have large numbers of machines on their network and rather than having to cut and paste a lot of different configurations together you can use wildcards in your ~/.ssh/config file.

However this is not amazingly easy – as SSH configuration requires the most general settings to be at the bottom of the file. So here’s a typical setup I might use for an imaginary client called abc: