These may not be for you but they worked perfect for me... The assy slips over the original mirror and is a tight fit molded for your mirror. Once you have it in position then you can turn a large knob on the back of the assy to tightened everything down real snug. On my F150 they were real tight and did not show any vibrations what-so-ever. I think the cost was $35 per side...
These are custom designed for your vehicle mirror.
http://www.cipausa.com/product/custo...g-mirror-11301
http://www.cipausa.com/sites/default...tion/11300.pdf
I was very pleased with how it fit my F150 mirrors - good rear view with no vibrations. It was a very snug fit. The large opening also allows your existing mirror to be used as well...
Looking at the list of Toyota vehicles I did not see the Highlander listed but looking at photos it looks like the Highlander and the Sequoia use similar mirrors. You may want to go to PEPBOYs and see if the Sequoia unit will fit your Highlander mirror before buying one.
This photo from google shows the KNOB mounting hole on the right side pretty good where you snug the whole assy in place. AMAZON also has a nice write-up and several reviews for this mirror. AMAZON also describes more on how the WEDGE STRIP locks down the mirror real snug when tightened up by the large KNOB...
http://www.amazon.com/CIPA-10000-Che.../dp/B0006H8ABI
This depiction shows the angle of view you will probably need to have using the rear view mirror for towing trailers.
I looked at the mirror that attached to the edges of the original installed mirrors using straps to hold them in place and didnt like them at all. This CIPA model slides over the whole rear side of the original mirror assy and fits like a glove leaving the glass part of the original mirror still in use.
Hope it works out ok for your installation...
Roy Ken