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I've got a box of those, not that it's any help to you right now.
You want something solid underneath really because as people walk around the boards will flex slightly and I guess this will over time have a crushing or compacting effect.
I'm guessing you have maybe up to an inch of space below the boards? If you've got something solid like a roof tile you could bed it into some mortar and rest the boards on that but it's probably overkill and it's still going to be quite hard to get it snug against the boards when you lay them on top
They're already sitting quite flat and there's quite a lot of rubble I took out from under the hearth that I can pack under it.
I could probably do with some of those little plastic wedge things to pack them in as well.