Keepers kicking in triangles can have a hard time keeping the ball in play long enough to get the number of repetitions needed for the drill to be productive. Having said that, it is physically possible for more skilled keepers to kick in a triangle and that way they all have to work on changing the angle of the ball. It's important that you have keepers practice kicking balls from where they'll come in a game. If you want a keeper to work on coming up to clear crosses from the sides, start the ball from the wing, not at the edge of the circle. If you have a ball machine, set it up out that wide, if not have a player hitting crosses from about five yards in from the sideline.
Footwork is key in kicking.
If I set balls up too close to a keeper, I don't reproduce the situation I want to incorporate in their game. I want my keeper to step up to the ball and clear accurately. The gate set-up with the ball on the left, flip for right. I like to give a keeper targets to clear to. We'll set up cones four yards apart so that there are two or three gates they can clear through around the edge of the circle.