Your intention is very clear. I know of no particular word, but one of the following would convey your message: hard surface, support, (or something specific as you have already mentioned).
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CliveWhat would you say instead?Something inane, no doubt. I would be tongue-tied.
ClivePS -
Maybe it's just a British thing.
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The cost for the day will be £10 to include coffee/tea.Please bring simple food to share.Please bring writing materials and something to lean on (clip-board or equivalent.)