Hey all. Figured I'd use this thread to post my personal example of this set-up in a DIY fashion.
I welded my Rola Basket with extension into one piece, then welded slotted tabs onto the bottom. The slots allow a little lateral adjustment and to allow all the tabs and angle brackets to be the same. I used 3/16" angle iron mounted to the factory rails using carriage bolts with a welded captive nut on the bottom (since it's hard to get to in that spot!).
I used galvanized bolts since stainless is expensive and it seems to be holding up fine. You'll see the angle iron brackets are tapered towards the inside of the basket- this is because the basket sits low enough on them that it would cause interference.
I used the old rubber tubing that comes with the basket/extension as a vibration isolator between the tabs and the angle iron brackets. The bolts holding the basket on use a lock washer to mitigate against backing out.
Everything is super sturdy and allows me to easily unbolt the basket when not needed leaving me with a nice clean vehicle! The angle iron brackets stay mounted to the rails and I thread in a spare bolt and antisieze to keep the threads in the captive nut safe from corrosion.
Pics below, but if you want to see a little more detail you can watch this silly video I put together...
Remounting the 4Runner roof basket! Christmas Tree Hunting Part 1 - YouTube