MGA - boot rack bag security?

[5MUG]

Hello all
we are venturing forth to Laon soon and plan to put our luggage on the boot rack as space is limited in the boot.

Does anyone have any top tips as to how we could secure the bag when we stop on the way down/back at say, a supermarket etc.?

I do not anticipate us having to leave it more than once or twice as we will decamp everything into the hotel when we arrive.

Also does anyone have any pointers as to a waterproof cover? I've had a look online and there are various covers etc but just wondered if anyone had anything to recommend?

Thanks
S