OK, i had this exact set up on my 29g for months before I got the very advanced plant bug and upgraded to a 4 bulb Aquatic-Life. Here's what you do:
1. Take the brackets and cut off the side with the screws in it even with part that sits on the tank's top edge. I put mine in a vise and used a hack saw. You could also use a dremmel moto-tool with a cutting wheel. You can lightly sand the cut edge to smooth if you want to.
2. Reverse the brackets. Slide the brackets into the slot with the remaining down side on the outside of the tank and slide them tight up against each side of the tank with the light centered where you want it.
3. Mark the spot where the tabs sit in the grooves.
4. Take fixture off and put upside down. Put brackets right where the marks are and spot with silicone cement. Let dry. The silicone will hold it well enough but not so much that you can't break it loose if you ever need to.
5. Reposition onto the tank.
You will not be able to use the tilt back feature, but that really is a gimmick as it stll doesn't give you enough room to work other than just feeding. I slid mine back enough to raise the front of the versa top then slid back center when done.
You have a great light for a twenty. Make sure you have FW bulbs. I used one full daylight and one flora-max in mine.
If you have any questions, just ask and I will help you through this if you decide to do it this way. Good luck, OS.