If your not going to drill use a 3100 Rio powerhead. I know most frown against this but the mag drive may rust if fully submerssed. Other than that just experiment until you find something that works. One thing about this hobby is that there is more than one way to skin a cat. There are no absolutes. I think your idea of a 10g submerssed into a larger tank is a good start. Another idea would be to use a 20g long. Get a couple of those small plastic rodent containers. You know, the ones with the ventilated top? Place one at each end of the 20g. One would serve as the area that the water from your overflow comes in, and in the other, you would place your return pump. In between the boxes you could put your mud and caulerpa. The boxes would serve as a means of protecting your pump from detritus, and the other box would allow for a water break so that it isn't constantly stirring up matter. Not the most desirable method, but it would work.