At the moment, your only real options are Pennine, themselves, or Tent Valeting Services. Unfortunately, I think Pennine only have the grey acrylic ones and the old canvas green ones in, but always worth a word. The canvas ones go for around £250 a pair.
The only other option, at the moment, is Tent Valeting Services, in Bolton, who will make them for you, for a similar cost.
Only other option is eBay etc, and a shed load of patience. There isn't actually, anywhere you can go and just buy these off the shelf, unfortunately. Undercover CI, who also make camper covers are looking at doing skirts soon, but are still trying to get templates together for all so not, currently available.
------------- The one good thing about being wrong is the joy it brings to others..
I have the brown canvas on our Pullman, replacing the green canvas about 3years ago but I kept my green bedskirts and still use them not bothered how it looks to others so long as they do their job, so maybe look for green ones as you may find them cheaper.
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