Broken knockout in panel - fix?

Hi all:
In the main breaker panel in my house, one of the larger cables (100A to sub in detached garage) exits though the bottom. The "installer" used a PVC bushing as a strain relief in the knockout, so there is no clamp. Also, the knockout used has a couple of larger-sized, concentric rings remaining, one of which has broken loose from the bottom of the panel. The result is that the cable passes through a plastic bushing that is just hanging onto the edge of a too-large hole in the bottom of the panel. There is nothing to keep this from falling through the hole and allowing the cable to chafe against the sharp metal edge of the panel knockout.
The only strain-reliefs I am familiar with are the metal ones that use a conduit nut on one side and a two-screw clamp on the other. I would like to replace the existing bushing with a proper clamping strain- relief of the correct size for the hole, but this will require disconnecting the circuit from the breaker and cutting an access hole in the wall below the panel. The wall has a complex hand-painted design that I would like to preserve. Access from the other side of the wall is possible, but presents it's own set of problems. Does anyone make a strain-relief that can be assembled around the cable and clamped into the hole from the inside of the panel? Thanks in advance.
Regards, John.
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If your main concern is the bushing falling through the too large knockout, you can get a washer, called a donut, which will fit over the threads of the bushing and prevent it from falling through. You'll still have to disconnect the wires and locknut to slip the donut on. Also , Arlington industries makes a snap in bushing called a "button", which may work
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the_tool_man wrote: ...

I don't have a link offhand to a picture, but there are 2-piece clamps. In essence it's two pieces that look like mirror images of each other--the one like the loose piece on the clamp style you're think of, and another it's mate.
W/ them, if there's room in the hole around the cable you may even be able to do it from outside the box, otherwise you just slide the two halve in from the top side of the open box and screw them back together again.
I've even manufactured something similar in a pinch rather than drive to town...
--
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Your'e going to let the "installer" get away with that? I wouldn't.
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On Wed, 28 Jan 2009 13:15:30 -0800 (PST), the_tool_man

Buy a cable clamp connector that fits the hole you have when the broken concentric ring is out of there.
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