I ordered the same extenders directly from Kouba link. They were great to work with. Immediate response to my inquiry and no charge for shipping.
Their site did not indicate that they made shorter links but you might be able to email them and see if they can work something up.
To tighten them down i just screwed the mirrors into the extender and use the mirror stalk to completely tighten the extender into the mount.
Just tested them out for the first time and they make a really big difference.