What is the most efficient symmetrical airfoil shape?
Truncation (chopping off the rear portion of the airfoil) does not reduce drag, although reducing up to 20% of the chord does not significantly increase it.  As Peter says, "trade off between more separation and more wetted surface area.  Hence "boat tail" bullets.

The 3:1 ratio of chord to width is apparently a bike racing specification.  Nothing new here,
the full airfoil produces the lowest drag.  Hence the cone they put on the back of the Space Shuttle while it was piggybacked on its 747.

What they did to meet the 3:1 specification was to truncate 66% of a 9 to 1 airfoil. 
This produces the lowest drag within the specification.

The Kamm tail shape was designed for automobiles, providing a weight savings (as racers constantly accelerate and deccelerate) and to make the vehicle a little shorter, which can help changing lanes in a crowd.  Two different applications.  But a fully symmetrical airfoil, such as the struts of a Cessna 172 will do the job.

For an aircraft strut, if strength allows, make it as long and thin as possible until drag values begin to rise.  9 to 1 rings a bell from boat hull design, and would be a good place to start.