I haven't seen one done in mesh yet. You do have the correct idea that you need to make sure it doesn't short anything out. You will need some form of standoff, or at the least something on the back of the mesh to isolate it. Craft shops carry stuff called "fun foam" that can be used for this, and it comes in a variety of colors to suit your theme. As for mounting, if you're keeping it lower profile, you can use 3M double sided foam tape.
If you want to keep the mesh open and retain airflow around the board, I would recommend screwing the motherboard to the standoffs with --additional standoffs-- then mount the mesh with regular board mount screws.
The real trick is going to be planning and cutting out the mesh. I'd recommend using a sheet of clear acetate or some other rigid but transparent or translucent material that is easily cut with a knife. Place it over the BACK of the board, trace your mount holes and board size. Cut to size, punch the holes. Attach two corners on one edge of the board with standoffs and screws. Start tracing around anything that sticks up too far. PCIe slots, Aux power connector, memory slots, CPU slot and area where cooler will go, heatsinks, fans, larger capacitors, ports and connectors. Keep marking and cutting until that sheet of film will lay flat.
Use the film to transfer all the measurements to your mesh. Cut the mesh carefully to avoid causing it to bend or deform. I'd recommend trying different cutting methods on a piece of scrap. Kinds of depends on the mesh you're using whether to use a Dremel and cutoff wheel, or something like a small pair of diagonal cutting pliers.
If you'd like to add some additional support for the mesh, consider leaving extra material along the longer edges of some of your cutouts and bending the mesh downward to form a stiffener. It doesn't have to go all the way to where it would contact the motherboard, merely 1/16" to 1/8". Best method is to clamp the part along the edge of a piece of MDF or other wood with a nice sharp 90 degree straight edge. Use another piece of wood clamped over the top to add support. Use a wood block, hammer or just about any other tool that will work to slowly fold down the edge. Literally just tap it enough to bend it down slightly, then move 1/2" along the edge and hit it again. Repeat all the way along the edge, then repeat the process until it "bottoms out" on the side of the piece of wood.