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

[Doofus]

What sort of bag? I used to use a vintsge suitcase, flexible varnish to make it waterproof, and then secured it to the rack using shackle bolts. Put your squishy bag(s) in the suitcase and you're sorted.

ETA: The suitcase appeared, to the casual observer, to have two leather straps around it, but in fact they concealed steel cable, padlocked underneath.