| You can use a solid limb (3 to 4 metres high as necessary) as the centre pick up point for the spiders web, then run a rope from that point back in to higher in the tree(trunk) to give it more strength if needed. 
 As for the plan, I'm building it like a net, the smallest square a hand width, then making the squares gradually bigger from the centre so that it forms a pyramid. 3 metres high and each side at the base 3 metres long with pegs to hold it's shape, can add or subtract depending on height of tree and the age of the climbers.
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