It would be useful if the rectangle had no borderline per default. Or default properties could be set up in preferences.
as a workaround, you can currently use the "New Transparent Cell" script in Antetype. If it's not available in the Develop menu, you can find it at our script repository at https://github.com/Antetype/Scripts. Just copy it into the scripts folder linked within the Develop menu.
If you can't see the Develop menu, you can enable it within the preferences: http://www.antetype.com/blog/2015/02/the-develop-menu/.
If the script is visible within the Develop menu, you can now add a shortcut via the system preferences to execute the "New Transparent Cell" script which will create... a new transparent cell without a border ;)
Hope that helps for the time being!