I no longer use a fid for burying, I use stainless wire. Starting with around 1m50 of wire bend it over onto itself, crimp the free ends toguether and then slide a plastic ball with the small hole toward the crimp. To do a bury slide the wire inside the cover from far back, exit where you want to finish the eye, put the free end in between the two wires, pull it back using the plastic ball to bunch up the cover.

A wire is the only way to deal with a 3mm dynnema line anyway as the smallest fid only works to 4mm.