I looked at a variety of different options for this, and eventually lost interest because there didn't seem to be anything really good quality / particularly fit for purpose. General consensus somewhere else on the forum was that the Fiamma or Thule ones will flap around at motorway speeds unless strapped down with an elaborate spider's web of bungee cords, so I discounted them.
This Skinz thing looked good but I emailed their UK distributor and didn't get a reply:
The Polaris bike rug is nice and simple but only for one bike and doesn't leave much on show to clamp the rack to:
Default Description
www.polaris-bikewear.co.uk
Velosock looks absolutely perfect, but is £180 and only suitable for one bike:
VELOSOCK indoor full bike cover for long-term bicycle storage in the garage or apartment. Thanks to its water-repellent fabric, this cover can also be used for transportation.
velosock.com
My normal BikeParka is great for storage off a bike rack. The 'Urban' model has 'lock-through slots' which you might be able to fudge a bike clamp arm through, but only takes handlebars up to 60cm wide (you need the 'XL' for anything wider). It's also only a solution for a single bike:
Discover the ultimate solution for secure and convenient bike covers with BikeParka. Our innovative bike covers provide protection against the elements, ensuring your bike stays in top condition. Explore our range for stylish and practical solutions to keep your two-wheeled companion safe.
bikeparka.com
I did wonder about just buying something cheap and cutting holes in it, but I suspect any areas of compromised structural integrity would soon be blown apart at 70mph:
£14.99 - BIKE STORAGE - Protective Bike Cover - 1 Bike - BTWIN
www.decathlon.co.uk
So sorry I don't have a solution, but hopefully this might save you time going around the houses investigating all of the candidates like I did!