I want to build an OTS with a large wing 60" X 12".  The designer mentioned that torsional stability came from the silk covering on the wing.  I do not do dope! Yes, I am a heretic. I use some form of shrinky stuff even on OTS.
The best way to achieve torsional strength is:

 Geodetic ribs?

 Cap strips?

 Balsa former between the ribs connecting the spars?

 Shrinkable fabric instead of plastic film?

I fly a Ringmaster with film covering, and I know of a lot of film-covered Rings.  It seems to have decent torsional stiffness -- and with the way the Ring wing is put together, it's as floppy as a noodle until you put covering on it.

I would use the regular stuff, and not any super-light stuff.

Geodetic ribs?

 Cap strips?

 Balsa former between the ribs connecting the spars?

Two notes:

One, your "balsa former between the ribs" is Don Hutchinson's shear webs.  These only work if there's leading edge sheeting: they make the open tube of the "C" web that Don mentions into a closed tube (a "D" tube structure); this closed tube is torsionally stiff.

Two, if you're going to fly in competition, per PAMPA's OTS rules you cannot make any modifications that change the aerodynamics of the airplane.  I've asked about things like this, and the answer is that extra ribs, cap strips, sheeted wings where the original was open construction, etc., are verboten.  PAMPA has a list of allowable modifications to the aircraft, and changes to the underlying structure to make the plane stronger are most definitely in that list, while changes that affect the outer shape of the aircraft are explicitly ruled out.  So anything you do that doesn't touch the covering is OK.

In theory, this means that if you want to build a Ringmaster with a carbon fiber leading edge and a honkin' big carbon fiber tube spar in the middle instead of what's there* then more power to you, as long as the changes do not show on the surface of the covering.

* the location seems to be specifically chosen as being the least effective place to put wood -- I'm just not sure what the designer was thinking.

Listen to Tim. He is fairly torsionally stable.

the jury is still out on his longitudinal stability however

Mark,

Well, there is that.