Tarp and dome, approximate relative sizes.
|You can narrow the top (once the tarp's bottom edge is attached to the dome) by taking one large pleat on each side at the top. You end up with a trapazoid. Use multiple trapazoids to cover your dome.|
You can use some bungie cords and/or rope and/or bungie balls to secure
the two closest grommets of the top edge and the two corner grommets with
forces in opposite directions, so that the top tarps hold their positions.
When tarps cross each other, you can attach to any of these points: