Fixed that. The command in the instructions had:
vagrant box add --name cdkv2 ~/Downloads/rhel-cdk-kubernetes-7.2.x86_64.vagrant-virtualbox.box*

instead of
vagrant box add --name cdkv2 ~/Downloads/rhel-cdk-kubernetes-7.2-29.x86_64.vagrant-virtualbox.box

So both the build was missing and the * was not required.