Balto -- that is a difficult situation, especially if it is a small wiggle. My first suggestion would be to try and determine what part is responsible for the jiggle.

If I understand your description correctly, I would guess the jiggle comes from the connection between the pipe clamp around your light and the L-shaped aluminum angle. If that is the problem maybe you could put a large U-bolt over your PVC light and down through the angle iron piece. Tightening that down should eliminate that jiggle.

If the jiggle is coming from elsewhere then you might consider using aluminum angle iron for everything since that will be stiffer than flat bar stock. I hope that helps, but if not send along a pic so we can make better guesses.