Carbon Monoxide (CO) has been my method of choice. And I was actually thinking of combining it with SN.
However recently I've been considering a switch from CO to hydrogen sulfide (H2S) gas, because the necessary gas concentration is 10x less than that of CO and it's easier to make. However if I do that then I can't use SN at the same time anymore, because SN is a known antidote for H2S.
How interesting isn't it? That a poison can act at the same time as antidote to another poison.
One other possibility is to combine H2S and cyanide (which I can make). Cyanide is supposed to be painful, however H2S is known to cause loss of consciousness pretty fast ("Knockdown effect", within few breaths), so in theory I wouldn't feel a thing from the cyanide. And the upside to this is that cyanide is surefire deadly, so no way I would fail.
I wish there was a way to test this "knockdown effect" of H2S on myself before actually using it to ctb. However this is completely impossible, since even low level exposure to H2S frequently leads to chronic sequelae.