I have not seen a bed rug type liner for rambox. I am unfamiliar with the beds with ramboxes. I assume the bottom patterns of the beds are the same. Could possibly buy the bedrug that is just the bottom peice and cut it to fit between ramboxes then velcro down.
I would give them a call or call a local distributor & ask them to check into it, the website may not be up to date
they do make them for the shorter bed with Rambox, maybe they also have them for the longer bed
Bedrug Bedrug Dodge Ram Bed w/ Rambox
Dodge Ram 09+ 5.7' Bed W/ Rambox
I spoke to BedRug a few months ago, and they said they aren't going to make one for the 6'4" Rambox… I explained to them that there is a rapidly growing market for these, but they couldn't care less.
You might want to call Bedrug and ask. I purchased a BedMat for my 2014 5'7" Rambox. Fits perfectly. Very happy. From their website it looks like it's not available for the 6'4" box but seems like another 9" in length would be easy for them to make. I purchased the bedmat because I thought the sides of the bedrug would limit the use of the bed separator/bed extender that comes with the Rambox.