A pretty well tested solution is making a "plywood sandwich" in your monobag, and putting your mono in between. Don't have to be that thick. So cut 2 roughly monofin shaped pieces that fit into your bag.
I usually also attach a "fragile - handle with care" sticker on it, but I'm not sure it does anything but provoke them
Sometimes we just put several fins in one bag and they generally seem to arrive in one piece.
However, the best solution I've seen so far was simply using a card board box - I'll have to attach a photo or it doesn't make much sense...
