The transparent plastic for the baggage at Heathrow?

Seeing the rule has not specified that the plastic thing needs to be the standard type they sell at the airport, why not get one of your own, as long as transparant. That way you can comfortably test it out, weighing it, chop & change what suits you better, in that bag or in the checked - suitin case. Not possible or very inconvenient to do that at the airport.