It seems like your drain line is connected to that white top. Instead of replacing it, you could also cut a square hole to make it look like this:
Then you could just put a light right over the the square hole.
You would also have to remove the filter pad tray, which you will lose your mechanical filtration. Is there anything in the chamber where the heater sits? Seems like a tight fit to add a skimmer in your sump. You would have to add it to the last chamber.
Another option is to set this wet/dry filter and use the money to set up a bigger sump. You will earn enough money to buy a 40g or 55g tank and add all the chambers. Most people build their sump/refugium around the skimmer.