google search a local timber merchant or sawmill....i found a price for a 2 x 2 for another project ( i know you dont need that thick ) ....turns out it was a little farm type shop, 8 ft long, was only £1.30 :)
When I did a similar project a few years back I used Treated roof tile batten, its rough sawn, its pressure treated and worked well for framing out. I recovered in white covered hard board. Most of it was put together with one of the "sticks like sh##" type of glues. Depend how much you want to spend, there's lots of specialist stuff out there. Try a builders merchant for the batten.
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If all is not lost where is it then?
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