for rhel builds I signed up for a free rhel account, downloaded their iso, and used bento's template to build the vagrant box.
only modification I made was this:
"mirror": "file:///path_to_bento_src_parent_dir/bento/packer_templates/"
and then dropped my iso in the same folder as the packer template.
Hosting in S3 with presigned link.
activate by running these commands:
sudo subscription-manager register --username=admin --password=secret
sudo subscription-manager attach --autocurrently while writing this my experience was the vbox provider is broken for their most recent versions of rhel based distros.