In days gone by I used to use the Teledyne Philbrick 1701 or 1702, these were femtoamp bias current devices and truely amazing, but now only found in museums! Look them up.
The key to operating in these areas is as mentioned teflon insulators and also 'air gaps' (use of rigid flying wire) and importantly 'board cleanliness'.
Our procedure was deflux with commercial solvent, wash with distilled water (lab grade), clean with lab grade alcohol and dry with nitrogen gas jet. (probably repeated several times).
With this method we had electrometer circuits that when you unplugged the electrode, the analogue reading remained stable for hours, probably days!
In your pA region, you should be able to get excellent results with the guard rings and good cleaning steps. Get rid of flux residues, that is cruical or you may design a hygrometer!
Then I suggest cleaning with IPA and a clean brush a few times as a more normal workshop routine. Lots of people don't understand 'cleaning', disolving the dirt and grime and then leaving it to dry just spreads it around, you really do need to wash it away.
If you do venture into fA you can put a guard ring around the inputs but have the hole as a big slot, i.e. produce a large TPH with the leads floating in space and 'air wire' to the pins.