Our cover fits our two tanks snugly and its not attached. No worries that its going to blow off.
There should be three notches on the metal device that screws down to hold both tanks. What about putting it at their widest? That might move the tanks apart more to make a tighter fit with the cover.
If that doesn't work, I've seen trailers with a bungee cord holding the cover to the tray. It doesn't look very good but does the job.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD