I did a simple remix of an iPhone 6 Plus case I like. I'm sure a design exactly like this must already exist somewhere on Thingiverse, but I haven't found it and I know at least one other person that would like a plain design like this. Anyway, I did very little here, just got rid of a cute elephant-shaped hole.
FYI: I've printed this in PETG and it's not nearly flexible enough to get my phone in. So I guess ninjaflex is probably required.
Update 1: I made this by remixing another case (should be indicated in this thing page), that other was not listed as a remix, but I bet it is a remix of this: http://www.thingiverse.com/thing:466518, I doubt mine here is actually different from this other case. Nevertheless, it took me a long time to find that other one, so I'll leave mine up.