This blog gives a summary of how to use buildout when download.zope.org is down http://devblog.4teamwork.ch/blog/2013/06/06/download-dot-zope-dot-org-is-down-how-to-fix-buildout/ however it is specific to Plone 4.2.
How do I go about getting a similar cache for Plone 4.3.1 so that my buildout won't fail when download.zope.org is down?
In the case of this particular outage, you don't need a cache[1]: you need a valid extends =
target. I've just fixed my Plone 4.3 buildout to avoid download.zope.org[2]. This should work for you:
[buildout]
extends = https://raw.github.com/plock/pins/master/plone-4-3
[plone]
#eggs +=
[1] Because Plone extends configuration files located on download.zope.org: http://dist.plone.org/release/4.3.1/versions.cfg
[2] As soon as a find the appropriate Zope configuration files, I'll fix 4.3.1 too.