If you're expecting several 100 MHz interference bandwidth (possible with fast diodes. e.g. schottky), why not measuring
with full oscilloscope bandwidth?
You have to use your FET proble with a short ground tip to get an almost valid measurement. You can measure e.g.
directly across a SMD filter cap and you should also determine the amount of ground bounce still effective at your
probe my measuring ground against ground. If you can assume, that you're measuring actual interferences, and
they are still too high directly at the filter cap, you obviously should reduce it's series inductance.
If you're already using small ceramic caps, the only practical way is to use a n-fold parallel circuit. Or accept the interference
at the output node and and add a low pass filter. But if your converter is injecting the interfering currents in the (common)ground
plane due to an unsuitable layout, no filter can help. A local ground plane for the converter is the best method to avoid this issue.
I think, it's mandatory for a 10 MHz converter.