Without actually doing test it's hard to know, however I believe that having a large amount of surface scum will affect water chemistry in such a small tank.
My idea for an overflow for you is to put a single piece of PVC (or a black box, whatever) into the centre of the tank. You can put your powerheads and heater inside of this.
This centre (non-glass hugging) overflow box will accomplish a few things.
-Hide all of your equipment without drilling.
- Skim the surface waters.
- Allow you to easily add more powerheads (all you will see is the outlet of the powerhead)
- Preserve all 4 glass sides of the tank for easy viewing.
- Preserve the tank for future FW use
-Give you a place to put sponges for some mechancial filtration
You could then cover (stack around, or glue) the PVC with rocks all the way to the surface water.
Here is a cheap MS paint mock up of what I mean...
I suppose the issue now would be the power supply. Somehow the cords are going to have to get inside. Also, your rock-scaping will be somewhat dependant on the position and shape of this overflow.
Anyways, thats just my 2 cents. Not trying to tell you what to do though. Good luck!