Here is what I made for on top of my Backflip tonneau cover. I have Bullring Shorty Low Profile tie downs in each corner. I put the girls smaller bikes in the middle and my wife's and mine on the outside with the rear wheels of our bikes tied together. Then a cable lock through the frames/rear wheels to the front tie downs to make sure nobody tries to steal them. Then tie downs going to the rear corner tie down points from the rear wheels/frames. Used it this summer on a trip and it worked great. All you need is a 6' piece of square 1" aluminum tubing, 4 front fork clamps, end caps for the square tubing, rubber bumpers for the bottom of the tubing for extra support in the middle, a can of plastidip, and 2 longer bolts to replace the Bakflip ones to bolt it down. Very easy and cheap to build. You can still use the tonneau cover when open all the way as well, just have to lift the final part over the bike rack since it pretty much sits where the cover usually does when opened up.