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I'm sorry CC99 - I didn't realise you were actually waiting for me to explain this.
In all the time I've been using one it's never really been a problem. Ususally by the time you get back to the car, house of whatever most of the mud off the bottom of the boot has worn off and it's just the shit up the sides, round the edges and over the top that's a problem. If you're still standing in the muck when you're trying to get your wellies off you're going to be standing in it anyway.
But in any event, to answer your question, all you need is a couple of sheets of newsp