Gravel Boards of the concrete variety about the same price as
scaffolding boards:
http://www.wickes.co.uk/Wickes-Reces...830mm/p/540064
Not so high but more stable and probably better at holding down any
sheet on account of its weight.

Concrete Gravel Boards would last much better, but twice the price,
and not as flexible, as in being cut to size, and to move around and
even being delivered. And then do you want in some fastenings, (screws
or nails)
'Horses for courses' so lots to think about, as you are planning for
years of use.