We went to REI and picked up a box.
We almost went with the Thule because it uses a two prong camping mechanism. I expected that this would fit between the slats easily. Unfortunately the bottom of the Thule box is not flat. It is designed to span a rack with only two cross bars. Each end of the box hangs down about 1.5 inches. You can see this if you look carefully at this photo.
Since the Frontrunner rack has a continuous set of slats, there is no place for this lower bulge to hang over. The rack would rest on that part.
I decided that this would put too much stress on the bottom of the box - the box walls are not very think for either Thule or Yakima. I also was not sure the clamps would reach down the extra distance to be able to get to the slats.
So, we picked up the Yakima Sky box 16.
Yakima uses a single sided clamp:
Sorry for the big pict, blame etrailer.com
Anyway, that clamp is long enough that it will not fit between the slats dropping straight down. It opens pretty wide by pivoting down, so when it is fully open you can wiggle and jiggle it into place. I was afraid that it would be a very fiddly operation, but it only took about 5-10 minutes. It will be easier the next time because I have the clamps properly spaced for the slats now.
It was very easy to remove - maybe 3 minutes.
The box is 81" long. Yakima's larger boxes are 92" long and I think there would be room for one, but it would extend over the front of the rack and I think it would be noisy.