Sorry, John, I think you're in danger of throwing additional equipment at a problem which your scope is perfectly capable of solving.
Set your scope to trigger on the rising edge of the clock, and set its time base such that you get one complete period on the screen, with the trigger point near the left edge of the graticule. Set the vertical scale and position such that the mid-point of the rising edge (ie. the trigger threshold) is in the middle of the screen, and such that the signal occupies most of the height of the display.
Set one X cursor precisely at t = 0, and the other cursor on the next rising edge, which should be near the right edge of the display.
Now, adjust the horizontal position and time base settings so you can see that second rising edge in detail, ie. so it completely fills the display. It should look like a diagonal line from bottom left to top right. Using this zoomed-in view, position the second X cursor as precisely as you can on the point where the trace crosses the trigger threshold.
This should give you a much more accurate indication of the period of one cycle.
If the trace isn't steady, then this indicates either noise or the deliberate use of spread-spectrum techniques. In this case, try turning on display persistence to see the extent of the variability, and take measurements to each extremity.
Another technique you can use is to measure over multiple periods. So, rather than placing the second X cursor on a point which is one cycle after the trigger, put it on, say, the 10th rising edge. As before, zoom right in to that edge, place the second cursor as precisely as you can on the point where the rising edge crosses the trigger point, then divide the measured X distance by 10.