I spent a few hours yesterday playing around with some gaffa, a broom, a Rig 3.5, pseudo catenary curves and then a dust sheet. Result: I think I've got a template for a DIY tarp that'll cope with the shape of the Seedhouse:
Just having the curve along the top ridge massively improves how the tarp hangs.
The main aim was to cover the door, but I think this'll also be big enough to work as somewhere to sit out of sun/rain and shelter whilst cooking, too. If my calculations are correct,
when this baby hits 88 miles per hour I'll just be able to squeak it out of a single width of fabric.
Hunting around for a suitable material to make it out of now, then it'll be adventures with sealants and sewing technical fabrics!