I don't know much about freezers, but that won't stop me from offering
My refrigerator door wouldn't stay shut. We paid the service man $100 to
replace the hinges. After I saw how easy the fix was, I felt rather silly
about not having done it myself.
Our freezer frequently will not stay closed. The problem is always
something sticking out a bit too far; but I am sure you have made sure that
is not the cause.