Place it on the table, place it in a static-free bag, place it in a static free bag on the floor, place it anywhere which will not damage it and is not near any static compoents, apart from that - you'll be fine.
You can fold a large lint free towel a couple times and
sit the mobo in the center of the towel.
You could also hang the mobo from the mounting holes
using a strong wire coat hanger. Hang it from two mounting
holes in the corners and not from any component on the board.
Hang it somewhere it wont be disturbed and wont be subject to swinging or human/animal traffic or moisture-no garage, maybe
a spare closet.
Cardboard is probably the hardest surface I would use because
you dont want to "POP" any solder joints.
If you use the coat hanger method I would wrap some electrical
tape around the ends that go through the board to help protect
the board. I use the coat hangers for cleaning boards that have
been in systems for years and every slot, nook and cranny need
to be rid of dust and dirt. I hang it like I was painting it, have 360
degrees of access and it makes cleaning it with air very easy and
the dust falls to the ground and not back into the case.