Aren't they rectangular?
yes but if you clip them together on the longer sides you get a square
Aren't they rectangular?
You could contact Neil at TW and see if he has a smaller one which isn't listed. Or he could do a custom job for a bit of an extra cost.
Already suggested that, and I am still posting here so I think her answer was obvious. Having said, knowing it would't work isn't the same as knowing how what to do to make it work. But I suspect there will be designs around.
I've never pitched one on a diagonal but from looking at then I suspect the way to make one would be to scallop the sides.
How about something like this?
The weight of a tarp (or anything else) depends on what it's made of. The porch tarp I linked weighs 6.5kg because it's done in something like 15oz cotton. In say, 4oz PU Polyester, it would be substantially lighter and likely cheaper too.