I had one in the old days, I rigged the lip with a plastic fitting to make it into a Venturi style pump. Basically you tap the lip of a maxi jet with a plastic tiny nipple that protrudes the circle on both sides. You add a airline tubing to the top of the nipple extended out of the tank, the amount of air can be controlled by another fitting on top to control the amount of air that is drawn in. This increases the amount of air bubbles in the skimmer body, and increases the skimming power.
I did the same on a CPR backpack and it Definitely increases skim dramatically. Why not get an external overflow and get a sump?