I agree with Vlad. The DRDT-2 plans specify tubes of 3"x6", 3/16" thick. If you look at the drdt2 website, there is a finite element analysis of it's design. You'll see that using the specified material, you'll get as much as .030" of bending...
That's using 3x6x3/16!
A regular c-frame upper section is designed to guide the pin, not hold it!