Several manufacturers now make a compatible tonneau for the MFT (no Rambox) without having to modify the rails or even swap out the bed braces for aftermarket (
rightonbrackets.com). There is an older thread dedicated to what works with the MFT (
MFT thread). The main issue is that the brackets used at the front of the bed to brace the sidewalls (due to the MFT) do not allow for traditional bed cover rails to fit properly.
I just did a lot of looking online for solutions and put on the Undercover Armor Flex hard tri-fold. I was interested in the Bak X4S and found where some have modified (cut) the rails to work with aftermarket bed brackets. I was not interested in spending $1k+ and then have to cut up the rails and purchase aftermarket brackets to make it work. The Truxedo Sentry and Sentry CT are similar hard roll ups and have MFT compatible models, but with a less secure (IMHO) locking system. I sent an inquiry to BAK and they had no plans to make a compatible X4S model. Anyway, below are some model numbers that are made for the MFT (no Rambox) 5.7' beds without modification. There may be more, but these were the main ones I was interested in.
Undercover ArmorFlex: AX32012
Undercover UltraFlex: UX32012
Undercover Flex: FX31012
BAK MX4: 448226
Truxedo Sentry CT: 1585816 (hard roll up)
Truxedo Sentry: 1585801
Also, the ArmorFlex works great and allows only a few drips here and there, mainly up front. I have not gotten around to sorting that out yet, but its not a huge issue for me. I cannot vouch for any of the others as this is my first bed cover. I purchased through Realtruck and called them for a much better price than listed online.
Good Luck