so i have an XC frame and fork that i'd like to put together. they are both take a standard 1 1/8" headset. the problem is that the steer tube is too short to use with any headsets i have.

i've read about "zero stack" (i know it goes by other names too) headsets, but my understanding so far has been that those always use a larger headtube diameter; ergo they aren't compatible with a standard 1 1/8" frame. is this correct? if not, what's out there?

provided those aren't an option, i think my lowest options are either Crank Bros directset or Extralite Head 2, both under 21mm.

"Zero stack" has nothing to do with the stack height of that headset, and is irrelevant. It is one such name that is used to describe an internal headset. They will not fit on your frame designed for a standard press in cups 1 1/8 headset. You might start by telling us exactly how much steerer there is to play with.