My suggestion would be to buy/salvage a dryer plug and cable.
Wire it to supply a two pole fused disconnect (which actually used to be how most dryer receptacles were once fed)
Then wire the 6-20 receptacle off of the fuses. Buy 20A fuses, so that the receptacle is approprately protected.
If you need two 6-20 receptacles, then gang together two fused disconnects fed from the same 30/40 amp dryer circuit ( I say 30/40, because different areas regulate the overcurrent protection to the dryer receptacle differently).