If you need connectivity, why are fiber washers being used? (full disclosure, perhaps there's a code compliancy issue you're trying to resolve with the washer I'm unaware of).
EDIT: Missed the IP part in the original question... my bad.
If the fiber washers are there for IP protection to keep fluids out, then you probably need a conductive gasket around the washer to make the electrical contact, or a counterbore ring for the washer so you can compress the washer adequately for IP protection, and then compress it a little more to make metal to metal contact. It can be done with the right materials and proper engineering.
I have seen it where the gland has a fibre washer between the gland and enclosure and inside the enclosure a earth ring is fitted then a lock nut but wasn’t sure what the general consensus was on good engineering practice?