you can try to replicate shallow water blackout but without water part, hyperventilate until you feel lightheaded then hold your breath until you pass out from the lack of oxygen. this method allows you to do this on bed for example so you won't fall painfully breaking something. it works because you overflow bloodstream with oxygen, which lowers your co2 level and when you hold your breath your body will eventually become starved of oxygen hence why blackout happens. if you would attempt this method without hyperventilation it won't work because co2 levels are too high from the beginning, they will rise too quickly to the point that it will force you to breath while still remaining conscious. though i don't know how long you will remain unconscious, 15-30 minutes seems unrealistic to me no matter what method to use