The steel bar can be dangerous if you have a passenger riding with you... Especially in an accident..
I made one using 1x2 woodstick.. hahaha... Drill 2 holes to slot into the headrest and 1 more for the bolt... hammer the bolt into it and it'll stay flushed..
Paint it if you want it to look better, otherwise, it's good to use...
The wood stick is theoretically less damaging since it would break more easily in the event of an accident compared to a steel or aluminum bar. And it's easier to work with too..