You are not supposed to put insulation inside the deforming part of the crimp where the electrical contact is made to my knowledge.
Fold the wire back over itself but it should be bare metal in there. Plastic can creep much more easily then copper.

What I did if its a
ridiculous fit is to make a solder junction between the wire with a thicker wire using good practices then crimp the good wire properly and just rely on the solder joint rather then trying to hack the reliability of a connector.
I am not convinced some kind of hack is going to offer better reliability then a solder joint and a proper crimp put together. It's going to be worse then just one crimp but I think it beats a hack.