Blind hole mount a 3.5" utility box?

Our garage was vandalised yesterday. Somebody ripped out a light fixture holding two flood lights, apparently just so they could then break the bulbs in the alley. The fixture, still attached to the 3.5" round plastic utility box, was left hanging by its wiring.
The fixture was mounted to the sofit on the underside of an eave overhang of the garage.
On examination, it looks like the guy who built the garage simply cut a round hole in the aluminum siding at that point and made a cunningly bent bracket out of sheet metal which somehow held the fixture in place for 10 years.
There's nothing but empty space behind the opening and I don't think there's any access to the space from inside the garage (not for normal sized humans anyway). The bracket that used to hold the fixture in place is pretty bent up, so I'm not sure I can duplicate the magic that was used in the original installation. The siding has the structural strength of a soda can, so I can get it back into shape with no problem.
Is there any "standard" technique for mounting a box in this situation?
--
Bert Hyman | St. Paul, MN | snipped-for-privacy@iphouse.com


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Old work ceiling box...
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snipped-for-privacy@goodone.com (Rick) wrote in

Boy; they make something for ->every situation, don't they?
Looks like that could work.
--
Bert Hyman | St. Paul, MN | snipped-for-privacy@iphouse.com


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Rick wrote:

But if there is nothing of substance for the old work box to clamp onto it is going to be a pretty wobbly installation. Although it would be pretty expensive it might be possible to insert one of the expandable support bars made for hanging ceiling fans through the opening and have it bridge the gap between the joists. The problem then might be whether it is possible to attach a standard ceiling box to the support if the existing opening happens to be far off center. Not having used one of these supports I don't know how their attachments work. Might be worth a look at one in the nearest borg though.
--
John McGaw
[Knoxville, TN, USA]
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John McGaw wrote:

I happened to put one of these in last weekend, for a ceiling fan. I got it at the big orange store for C$18 (so it'll be like $7.95 in the US, right?), branded Westinghouse no less. The bar has toothed disks on either end for grabbing wooden joists and extends out to (as I recall) 24". And once in place it has exactly the same connections as a standard ceiling box, except of course they're solid enough for an 80 lb ceiling fan.
So yeah, that would be my suggestion.
Chip C Toronto
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onto
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I had assumed since it was aluminum soffit there might be 1/4 plywood underneath. Fan mount brace would work against the rafter tails or maybe between the fascia and top plate...
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Cut some boards slightly longer than the hole. 2x4, 1x4, 1x3 whatever fits and whatever will build out to the appropriate depth to mount a box. Angle a board in the hole and screw it to the back wall, being careful not to use screws so long they protrude on the inside of the garage, then successively attach more boards till the depth is correct. Attach the electrical box, regular or pancake, to the board.
--
Luke
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