--buildout-download-base have to be treat same as --download-base.

bootstrap/bootstrap.py

@@ -79,7 +79,8 @@ def normalize_to_url(option, opt_str, value, parser):
             value = 'file://%s' % (
                 urllib.pathname2url(
                     os.path.abspath(os.path.expanduser(value))),)
-        if opt_str == '--download-base' and not value.endswith('/'):
+        if opt_str in ['--download-base', '--buildout-download-base'] \
+            and not value.endswith('/'):
             # Download base needs a trailing slash to make the world happy.
             value += '/'
     else: