How can I seal 8 inch round holes in my siding?
I would procure a sheet of aluminum or steel (available at home improvement and hardware stores), fold a suitable hem at the bottom, and slide it up behind both courses of siding (above and below the holes). Friction will probably keep it in place for the short term, but you could dab some silicone behind to lock it in.

|  |&lt;-- upper siding course
|  |  __
|  |||  |
|__|||  |&lt;-- lower siding course
    ||  |
    ||x |
    ||x | &lt;-- hole location
    ||x |
    ||  |
    ||__||
    |____|&lt;-- hemmed sheet


If you fit this well and paint it it could actually be a finished repair. Insulate behind first. Keep in mind that the top edge of the sheet will be exposed in the siding gap above, so you might seal that with caulk or lap another sheet all the way up to and behind the course above.