Hi Ross,
I don't exactly have any blueprints for building a dredge, but a while back
I did an article for American Conchologist on dredging, and the illustration
which accompanied that article can be seen here:
http://members.aol.com/paulcyp/XXDREDGEXX.JPG
The text of the article includes some information on constructing a
dredge. If you would be interested in that, I'll send it to you.
Regards,
Paul M.