Double buggy rain cover - how do 'universal' ones work?

[Lio]

We have been given a Bruin double buggy, but before I buy a mothercare 'one size supposedly fits all' rain cover, does anyone have one and does it work for you? How does it stay on?

...

befor you go to motherecare go to an independent baby shop and get one from there
i ve got a graco duo sport and it (raincover) was only 14.99 and i got it the next day where if id gone to m/care it would have cost me 30.00

thanks hitchcock