So, if I'm understanding the question correctly: can a trigger be shaped as anything other than a box/rectangle?
The answer is no. The collision detection used on triggers is kept to simple box shapes. Nothing more sophisticated than that.
The only instance where you can sort of make a trigger that isn't box shaped, is when you make a brush bound to a WordModel that when damaged by weapon fire will act as a trigger. You set BoxPhysics to False so that the game engine will follow the brush's actual outline rather than put a hitbox around it. And you enter a DamageCommand in the DamageProperties of the WorldModel to make something happen in reaction to the weapon damage.
If that's not a good substitute for what you had in mind, then you'll have to use multiple player touchable triggers placed in such a way that they resemble the shape you wanted to make. It will be like using small lego building blocks to make a larger shape, and having all the triggers do the same thing when touched. But it would be necessary to have the first trigger touched lock all the others, so that the same commands aren't sent multiple times.