Converting an external hard drive enclosure into a write blocker?

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Hey all,
I wanted to find out if there was a method to convert an external  
hard drive enclosure into a "cheap" write blocker device?  I'm not  
looking for something to use from a forensic standpoint.  Basically  
if I want to put a hard drive into an enclosure and pull data/burn  
data to DVD/whatever off of it, but prevent anything from being  
written to the drive, I can do that.

Thanks ahead of time,
Tom
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