Try an app called perfect app protector. It does many things, and most importantly it has an option for single app rotation lock. I only use it for youtube rotation lock. It also has password protection for apps, but I don't use that. I tried a few other rotation lock apps which ended up messing with the behaviour of other apps, and I ended up un installing them. This app was great because you can choose to edit the rotation settings of one app only (or as many as you like) leaving the other apps unharmed (unlike the global rotation lock apps). The only problem with perfect app protector right now is a bug that kills the rotation lock if you minimize youtube then come back to it again. I commented it about it in the play store review though, and I'm hopeful they will fix it in an update. If you find a similar/better app, let me know
Edit: Actually, after using this app for a while, I found it to be very glitchy, and full of bugs. They really need to clean this app up, in theory it's the perfect app.. So anyway, I un installed it, and I'm back to square 1. All we really need is a rotation lock app that works on the apps we choose only, rather than work globally. I found a couple other apps that have the same youtube rotation problem.. One of them is video editor, forgot what the other one was. Ughh.. So annoying really.