Are the metal rods threaded?

If so, put the threaded end into your drill and tighen it down. Then "drill" the non threaded end into the nylon rod. Keep drilling it in until the first part of the threaded end begins to thread into the sleeve. Let it thread in about a quarter inch or so. Then you'll screw your clevis onto the remaining threads.

Repeat the same thing on the other side, but this side might need to be tweaked as far as its length is concerned so that when the servo is centered, the surface is also centered. YOu would do this adjustment first my cutting the nylon rod to the right length. You can then vary the amount of metal threaded into the sleeve. And lastly make any minor adjustments by adjusting how far on the clevis is screwed.

Hope this helps.