The easiest and cheapest option is to buy a tarp from any good DIY store and fold it to fit.
Make sure it is a couple of inches smaller than your tent all the way round to prevent water pooling between them.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets