My first swing cup holder design was http://www.thingiverse.com/thing:339178 -- and it works well enough holding your drink off the stationary side-reinforcing bar of the swing. However, I marked it as a work-in-progress because it clearly could be improved.
The first improvement was to switch to a double-cup holder that is much taller to prevent spills and holds your drinks over the bar rather than next to it (an alignment notch on the bottom of the holder keeps it centered). That version, printed in natural PLA, is shown in the photos and the design is in swingcupholder20140527parts.stl .
Ah, but the sides would hit a mug handle, and the single-bolt clamp works, but isn't as nice as it could be. Thus, swingcupholder20140529parts.stl lowers the walls a bit (why not?), adds a cut-out for a mug handle, and uses two 1/4 20 bolts with a loose drop-in fit and hex head cut-out to keep them from turning as you put the nuts on underneath. I was pretty confident it would work better than the other two designs, although the old designs are now mounted on my older swing and seem to work pretty well. It is now confirmed: the gray ones on my new swing are the 20140529 design, and they are much nicer.
These improved double-cup holders are easy prints and can be single-wall with fill 20% or less. It took a while to print in PLA -- as much as 3 hours on my MakerGear M2 extruding 0.25mm. I had some trouble with the gray PLA lifting from the bed -- most PLA prints nicely on my M2's 70C heated borosilicate bed, but this Rosewill gray doesn't stick to glass as well as most. You'll also need a couple of 1/4 20 bolts and a nut or wingnut for each.