You could solder a pair of ears onto the sides of the 510 spring retaining barrel while it's apart. Bend them 90 out from the barrel and solder those to pretinned sections on the top plate, that would reduce the time the plastic washer is heated. A piece of wet paper town jammed into the 510 hole should keep the washer cool as well. Regular 60/40 solder would work for that no need for anything special. Just be aware of where you are soldering the ears to the top plate so it fits between the screw holes.