Support lip of bezel on soft wood block and by hand lean on a screwdriver or punch to fold/crimp a few spots behind the case.

For a fancier tool might look at glaziers pliers, they're wide smooth jaws that are parallel when opened some, not closed. Some come with plastic jaw pads.
You could narrow one jaw to just as wide as the spots you want to crimp.
http://distributionsvimac.com/Catalogue.php?CAT=A05-A&LNG=1

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-