I think there are a few factors involved, but airfoil and dihedral stand out for me.

The airfoil can produce very different amounts and type of lift during the outside loop. Mainly the arfoil is at a negative angle-of-attack the whole way through the manouvre and this is worst when pulling up from the bottom (inverted) part of the loop. A symmetric airfoil has no problem with this, but if the bottom of the airfoil is flat or concave you get a lot less lift when it is inverted. The non-symmetric airfoils are also more prone to stall while inverted. Also, the smaller the radius of the leading the worse the inverted stall becomes.

Diheral, on the other hand, will make the plane want to flip over and fly right-side-up. The wing is in an unstable lift state and will want to a move to the normal stable configuration. And bank angle will make the plane flop out of the outside loop in a cork-skrew fashion.

The best configuration for an outside loop would be a wing with a symmetric airfoil and no diheral.

A less important factor is the weight distribution in the model. If the plane has a high wing the weight is below the wing in normal flight. This creates a stable condition due to the pendulum effect. When inverted, the weight is on the inside of the loop and will tend to want to go to the outside due to angular momentum. Any bank while in the outside loop gives the weight a direction in which to flip the wing over so that the weight is on the outside of the loop.

So, the best configuration as far as weight goes is to have the wing mounted centrally,or even at the bottom of the fuselage.

-Q.