how to tie a knot you cant undo

[raychul]

I've tried ligatures before but not tight enough. what is the best knot that i can tie that will be tight enough for me to lose consciousness and strong enough to not come loose. ive tried slip knot but couldnt get it tight enough due to not being able to actually hang from anything (i did try kneeling down with it tied around a door handle but no luck)

 

[raychul]

follow up: I'm being discharged from hospital next week, so i will be able to use a slip knot when hanging. what is the best material to use to hang from a height?