I have an old drafting board that I use, and I wood screw the l/e piece down, then use push pins to hold the t/e, after sanding that thickness I turn the flap/stab or what ever I am sanding over and go to a smaller rod on the trailing edeg.
  Hope that this helps...

Get square key stock at the local builders supply and tack it down with ca.
Don

YEP - you can also tack down the rods with CA too.....

Apply strips of sandpaper with rubber cement to the mirror and this will minimize the sliding around of the piece you are trying to sand. The sandpaper will pull off and the cement residue can easily be cleaned by rubbing. Other adhesives could be used (might be hard to remove) or adhesive backed sandpaper could also be used.

 I perform this operation on my glass top workbench. This process goes a lot quicker than it sounds here...

 (1) Clean the glass so it's nice and smooth.

 (2) Put down a layer of 2" blue painters tape on the glass with the centerline of the tape roughly where the rods will be.

 (3) Lay the part to be sanded on top of the blue tape.

 (4) Place the desired thickness (music wire) rods in place along the edges of the part to be sanded. (I pre-cut the rods to extend past the ends of the part to be sanded on each end by a little bit.)

 (5) Tape the ends of the rods down to hold them in place temporarily.

 (6) Lift the part to be sanded out of the way and then CA the rods to the blue tape and wipe away any excess.

 (7) Lay the part back down between the rods and do your sanding as desired.

 When you are done you may end up sacrificing a bit of wire but you can peel the tape and everything away from your benchtop fairly easily.