The stray capacitances to the rest of the world will be of the same order as
the required value of capacitance. The presence of stray capacitances, not
being accounted for in the design, will cause whatever it is to behave in an
unpredictable manner. Your electrical "circuit" or model will be different
and more complicated than what you think it is.

Methinks you should re-engineer whatever it is to something which requires a
capacitance of something greater than 0.4 pF and is of a more realiseable
and predictable value.

Or better still, you may even re-design something to achieve the desired
performance which does not require a loading capacitor at all.
----
Reg.