Seeking for an alternative to lock the screen without pressing the power button in Lollipop I find it cumbersome to press the power button on my Android Lollipop device for locking the screen.

Are there alternatives (like a widget or a gesture) that use some interaction with the touch screen to get the device locked, apart from the automatic lock after timeout?

Also, is there any possibility to lock the screen without the need to install an app?
Try Screen Off and Lock and Screen Lock app from the playstore. You don't need to press the power button or wait for timeout to lock the screen. Just press the icon and it will lock the screen. You can customize the sounds and animation in which an animation plays before locking the screen.