I had a video of Quique S preforming a outside loop from inverted very low, with only one roll through out. This was years ago at a TOC and after watching the video I was determined to try it myself.
I find for a smooth round loop you need horsepower, moderate control throw but not to much, and a forward C.G not a 3d C.G.
practice with long slow rolls to start with then move on to rolling circles.
rolling loops are no different than rolling circles, except for the force of gravity squashing the top of the loop. At the top of the loop minimal rudder is needed.
When I started with circles and loops I used a stick plane at home with my transmitter imagnating the transmitter control inputs as the plane performs a circle or loop.

with loops it is best to start with the first half only.
Anthony