Hello Giorgio,
Here is the link to Neil Thurston and Thurston Canvas:
http://www.thurstonsails.com/Canvas.html
Since Thurston is J Boats' U.S. factory source for dodgers, they
already have the J/92 measurements. My dodger fit perfectly and was
less than half the cost of a locally-made custom dodger. A difference
is that your boat was made by J Boats France - the handrails and/or
sea hood may be slightly different. I would send Neil an e-mail and
ask if there are any key measurements he wants you to check.
Another consideration is the fasteners. Thurston uses "Lift-the-Dot"
fasteners on forward portions of the dodger, and snaps on the sides.
I found that the screw-in Lift-the-Dot studs snagged lines, and they
worked loose and allowed water into the balse-cored sea hood. I would
instead use Lift-the-Dot machine screw studs, overdrilled and epoxied
when installed. These are stronger and can be replaced with short
machine screws and cup washers during the frequent times that you
don't need the dodger.
Let me know if you have other questions.
Bob Johnston