If you have lost 2 fins out of the same board, it would seem to be quite apparent that the fins are not fitting in the fin box correctly.
There should never be any reason to put anything (fin cover material or otherwise) in a correctly fitting Powerbox (or Tuttle box/Deep Tuttle; Tige; Bic Power Trim box.)
The only sort of fin box that needs to be "shimmed" is an "A" or "E" box and then the shim is only to "tighten" the fin in the slot if one or the other is out of spec.
Correctly fitted, the fin bolt simply "DRAWS" the tapered surfaces into full contact.
The tapered surfaces then take all the stress and strain, and the fin bolt (bolts) simply enusres that the tapered surfaces remain in contact.
The reason your fin screw is/was 6 mm x 1.0 with a big #3 Phillips drive in the pan head is so that you would use a #3 Phillips screwdriver (normally supplied with nearly all F2's and Starboards) to tighten them.
Use of a lockwasher will not help if the tapers on your fin do not fit correctly.
Adding a lockwasher will ensure that air can get down the shank of the screw.
You need a seal under the bolt head and washer to seal the fin box from the atomosphere above the tail of the board.
Some times it is necessary to add a little material on/in the fin head tapers or the fin box tapers and carefully file the added material back until you get the good fit (with some "DRAW" to hold the fin tightly in the finbox.
I sailed Powerbox fins for 15-20 years and never lost a single fin.
If the fin "falls out" of your board when you remove the screw (s), then it simply does not fit properly.
If you have to smack it with your hand or with a soft mallet to get it out of the board (after removing the screw (s), then it fits correctly.