I just installed it on a Rebel with Ramboxes. I like the cover so far. I wish it overlapped the edges a little more.
Mine sagged inward, as well...I have to use a foam backer on it to level it out. The vendor I bought it from told me that the BakFlip wouldn't work with Ramboxes so the alternative was the UltraFlex. Then they asked me which tie down rails I had (black) so they could get me the correct cover. Long story short, they did not tell me I had to remove my rails completely to use this cover. Bummed about that, frankly. Apparently one model will not work at all and the other will work if you throw your black rails away, lol. I do think the cover is overpriced, though. It also did not come with the light due to shortages. I will update on the cover over the next few weeks.