My improvised, low-dollar, one size fits all, stuck cog removal rig:



1. Mark largest cog on 2x4s. Drill holes to accommodate screws.
2. Replace freewheel. Secure with screws.
3. Place chain whip appropriately. Slide circular end of 1" wrench over end of chain whip.
4. Stand on 2x4s straddling freewheel/chain whip assembly because you don't have anything to secure the 2x4s and hold them in place.
5. Hold wrench in left hand, thus holding chain whip in place for a quick application of force via rubber mallet (held in right hand).

I've sorted out some pretty wretched freewheels this way. None have yet defeated me. And in the absence of a bench vise (or a bench to house one) this will get you there. Ignore the laughter; focus on the end result.