Body timer does work for me. But obviously there are days where your mind immediately feels sluggish, and in those cases I would like to keep trying, at least for a bit (to see whether my mind is just trying to avoid meditation)
Also, over time, it takes longer and longer to reach a point where your body complains or your mind finds it difficult to continue. For me this point is typically already longer than 1 hour, so not really good if I want to do a shorter meditation.
Alarm I used in the past, but only a silent one on my phone (I don't want to wake up my partner)