In case you are travelling and cannot bring the bar with you ;-) a good alternative is the "prisoner squat". Place both hands behind you head crossing your fing

In case you are travelling and cannot bring the bar with you ;-) a good alternative is the "prisoner squat". Place both hands behind you head crossing your fingers. With elbows out as much as you can do the squat like prisoners do it, veeery sloooowly, start with 4 x 12.