Don't have a Workmate. Check out http://bluecollarmtb.com/2005/03/02/bike-repair-stand-blue-collar-style/
Step 1: Arms
One arm has two 3/8 dia holes drilled in it 6 1/4 inches from the end and 3/4 inches from the top and bottom.
Use these holes as a guide to drill the upright and other arm after everything is assembled.
Cut two pieces of 2x4 3 1/2 inches long for the spacer blocks.
Screw the arms to the spacer blocks so there's a gap in the arm assembly for the upright to fit through snugly.
Step 2: Upright
Cut two pieces of 2x4 15 inches long for the support blocks.
Attached the support blocks to the upright 2 inches from the bottom of the upright.
Note: As an alternative, attach the support blocks flush with the bottom of the upright, then the stand could be clamped to the edge of a workbench
Step 3: Final Assembly
Drill a series of evenly spaced holes down the upright so the height of the arms can be adjusted.
Attach the arms to the upright using two 4 inch long 3/8" carriage bolts, washers and wingnuts.
Clamp the whole thing in the Workmate as shown. You can also clamp the back of the support blocks down to the Workmate table for extra support.
Step 4: Holding Bike
If the bike needs to be held rigidly, put a C-clamp over the open end of the arms and tighten to clamp the seat post/seat tube between the arms.