The "professional" clamps made for this job are not very expensive.

http://buy1.snapon.com/catalog/item.asp?search=true&item_ID=66783&PartNo=ya2850a&group_id=1461&supersede=&store=snapon-store&tool=all

Try $7.25 for the pair.

It's more expensive in terms of your time to adapt something not made for the job.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles