In this example, we're going to download the Midnight Sun box.
# The box URL is https://app.vagrantup.com/uwmidsun/boxes/box/versions/2.1.0
# In general, we just take the box URL, and then append the provider URL
# https://app.vagrantup.com/<organization name>/boxes/<box name>/versions/<version>/providers/<provider>.box
wget https://app.vagrantup.com/uwmidsun/boxes/box/versions/2.1.0/providers/virtualbox.box -O box-2.1.0.box