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:

Any exposed strut.
A grommet on an already mounted tarp.
The base struts of the dome are always viable, even if you have to reach them with a length of rope. To cut synthetic rope:
wrap with tape, cut, burn ends with lighter until fibers fuse, remove tape.
To another bungie or rope that is itself secured to something.