I made some out of 1/4" acrtlic for my 48" CFL fixture over my 55g for the same reason. Here are some pictures, i kept it simple but effective.
This looks like a great way to use acrylic to prop up a light. The moisture exposure is the same on both sides of the vertical pieces, so they shouldn't warp significantly. The horizontal pieces are reinforced by the glued on vertical pieces, making them very resistant to warping. And, it looks great!