I have a cardboard that was intended for my z3, and it works good although the cardboard didn't sealed properly. So, I tried to use this cardboard with my Xperia L for YouTube, then I just realised that 360 videos on youtube didn't work, and also, there isn't a option where we could change the YouTube viewing mode to Cardboard, unless we're on 4.4+, andThe Cardboard app and those 360 apps don't work with Xperia L, too.
So the only option to enjoy cardboard with xperia l is with certain apps like "VR Player" app or something else to watch videos from our phone's files, or you could search something on youtube and try to filter your search by checking "3D", then there will be a side-by-side video that you could watch with your xperia L + Cardboard.
Of course the quality will be not good using Xperia L because of the screen pixel density and resolution. But it was decent for me, Even I watch my movies inside my phone with that VR Player app, yes it isn't great like the "real" 3D thing, but it is worth trying. I watched those movies with my Xperia L and Cardboard with VR Player app frequently, just to make my watching experiences a bit classy lol