You have many options here. The cheapest in my opinion would be to go to your local hardware store and buy the clip on round shop light and a 6500kelvin outdoor bulb that fits in there. You can then figure out a way to clip it so that the light presses flat against the side of the refuge. Many people have had good results in growing macroalgae this way including myself. This will run you about 15 dollars total about 8 dollars for the bulb, granted it may not be the best looking. You could probably put 2 on the back if the length is 19 inches.
Higher wattage strips would be better at a low kelvin level(around 6500k), but will be much more expensive for the fixture and the recurring costs of bulbs. Any 18 inch light strip will work, just get the highest wattage you can.