IMO drowning is the most reliable method with 100% success if done properly (not risking absorbing too small dose cause of unexpected vomiting, breaking of a rope after brain damage point, etc. ), because when an unconcious person is deep underwater then nothing can go wrong, except for being spotted while drowning and then being RESCUED. 
It is difficult to find deep water spot that is completely private and going to any other spot at 4 or 5 AM still has a risk.
So I have a setup for drowning in water under 1m depth that is accessible for many people in a private spot (ex. buying inflatable pool and set it inside a house, rent airnb with a shallow pool and maybe trying in a big bathtub if someone is really small person).
First will be needed 2x hinged cuffs or maybe budget zipcuffs to fully restrain your ankles very tight in X shape (so every movement will hurt and standing up will be very difficult) and another pair for cuffing your hands behind your knees also very tight in X shape so you can't move your hand behind your back or in front of you, then even if you stand up you won't be able to straighten up and lost 50% of your height. Now it be possible to drown in about 1 meter deep water.
Next will be needed dog collar that is used for strong big dogs (alternatively DIY steel collar used for BDSM related activities, but it need to be strong) and pair of normal handcuffs with short chain (hinged ones would be too short), then put collar on your neck and attach it to an ankle with handcuffs. Now it be possible to drown in water with depth under 50 centimeters.
Last needed item will be any backpack (use padlock to secure zippers from opening) with 5-10kg of lead bars(less expensive) or just buy these small lead weights from diving store, and 5kg steel kettlebell attached to hand or collar with another handcuff to make sure you won't start floating if something happens to backpack.
Let me know any other way to attach one's head to ankle if you have better idea.
It is difficult to find deep water spot that is completely private and going to any other spot at 4 or 5 AM still has a risk.
So I have a setup for drowning in water under 1m depth that is accessible for many people in a private spot (ex. buying inflatable pool and set it inside a house, rent airnb with a shallow pool and maybe trying in a big bathtub if someone is really small person).
First will be needed 2x hinged cuffs or maybe budget zipcuffs to fully restrain your ankles very tight in X shape (so every movement will hurt and standing up will be very difficult) and another pair for cuffing your hands behind your knees also very tight in X shape so you can't move your hand behind your back or in front of you, then even if you stand up you won't be able to straighten up and lost 50% of your height. Now it be possible to drown in about 1 meter deep water.
Next will be needed dog collar that is used for strong big dogs (alternatively DIY steel collar used for BDSM related activities, but it need to be strong) and pair of normal handcuffs with short chain (hinged ones would be too short), then put collar on your neck and attach it to an ankle with handcuffs. Now it be possible to drown in water with depth under 50 centimeters.
Last needed item will be any backpack (use padlock to secure zippers from opening) with 5-10kg of lead bars(less expensive) or just buy these small lead weights from diving store, and 5kg steel kettlebell attached to hand or collar with another handcuff to make sure you won't start floating if something happens to backpack.
Let me know any other way to attach one's head to ankle if you have better idea.
