Stick a piece of linoleum in the gap... or with a knife or chisel, take off a strip of wood from a scrap board and put that in the gap.

Heck, a thin strip of drywall wedged in that gap would do.

use a mastic tape to seal the gap between
sheetrock & supply box.
then angle the screw so that it catches.

do you have a drill with screw tip?
makes it much easier.

best of luck

If you feel around with an ice pick you will find where the screw made a hole in the metal flange that surounds the vent. If you have trouble finding the hole with screws,get longer screws so you grip screw with fingers while feeling for the hole.

you should be using a 2 1/2 to 3" screw.

have you managed it yet??

None of the suggestions would satisfy me at my house. The HVAC clowns had laid a scrap piece of duct drive coupling in the attic beside the narrow ends of the boot. Someone in the attic held it in place while a person in the room drove the register mounting screws through the metal scrap. Two problems: The registers were not parallel to the wall nor aligned with each other in the longer rooms plus the fact that removing the register left the pieces in the attic loose. Two people, one in the attic, would be required to remount the register.
I used Gorilla glue to hold wooden blocks replacing the scraps of metal. Registers could then be installed in alignment by one person.
An alternative would be sheet metal angles, perhaps 24 gauge, pop riveted to the boot, one leg of the angle into the boot and the other leg projecting out horizontally from the boot.

This post was edited by bus_driver on Mon, Aug 4, 14 at 19:18

Classic post and run...

"The HVAC clowns had laid a scrap piece of duct drive coupling in the attic beside the narrow ends of the boot. Someone in the attic held it in place while a person in the room drove the register mounting screws through the metal scrap."

Come on bus,giveum a break. Hvac is only a sideline ,they are drywallers by trade. I once visited a home where they had repaired some damaged sheet rock and were in the process of taping and bedding. They had tape on the floor applying mud to it then placing it on the wall and smoothing it out with their hands. BTW,can you tell me how to get the cover off my electric service box,I want to add some outlets in my kitchen.

I keep trying...... I tried wedging and gluing a thin piece of wood it there, it sort of worked for one of the registers, although the screw is at quite an angle. On the other one, in spite of the wood there is still a hole - at least a half inch wide where the screw was. A little over 3 inches up I poked at something, must have been metal, but my drill wouldn't penetrate it. If I were to go up in the attic (130 degrees, no flooring) I would probably never come down again - unless I passed out and fell through the ceiling! : ) I just bought a narrow piece of screen door molding. I am going to cut it to fit inside the register, screw it to the ceiling, then screw the register into the middle of the slat. I know that is really rigging, but maybe better than what the paint guy in HD said - Liquid Nails? Thanks for all your suggestions. My hubby (who is smart but not all a handy-man, keeps telling me I should hire someone to do all this. hmmmm

what kind of tools are you working with?
screwdriver?
drill?

what kind of screws?
metal screws?
wood screws?

wedging & gluing wood into the gap isn't
a lasting fix.
the supply register should screw
into the supply box in the attic.
if the supply box is 3" above the ceiling...
then it is installed incorrectly.
this leads to conditioned air leakage
at the supply box, which causes
condensation. wet sheetrock will
show water stains & crumble.

the person who installs duct & supply
boxes is low paid guy on hvac crew,
few know how to run duct properly
or install supply boxes.
so you'd have to really look for someone willing
to do this properly.

ideally supply box is nailed to 2x ceiling joist on one
side & framed with 1xs at minimum so that supply
grill can be screwed into wood by angling the
screw slightly.
this usually doesn't happen, so that leave screwing
supply grill into metal supply box. its hard.

busdriver...that they had two people working to
install supply boxes is suprising. using the drive
isn't a bad idea..its what they had on hand and
would keep supply boxes flat to attic side of
ceiling, usually the 3" screw will pick the supply
box off the attic floor...as I suspect has happened
at this lady's house.

sealing the supply box to the attic floor is
important. 30% of the air that we pay to
condition leaks into attic.
the things I see when working on ducts in
attics over the years are horrifying...there really
needs to be a lot of improvement in duct installs.

best of luck op.

Use of the drive in this instance was not good at all. For those who did not understand, the drive was held only by the register mounting screw. Removing the register to seal the boot to the cut edges of the drywall and to align the registers made it impossible for one person to reinstall the registers.
The clown in the attic who held the drive was the licensed person in that company, the "Qualified" person using the terms in the NC licensing law. The helper below, where I was, who was slapping up the registers in misalignment told me the drive was being fastened also to the boot.
When they broke for lunch, I went with flashlight into the attic to inspect. The drive pieces used did not touch the boot suitable for fastening by any method. And two runs of the flex duct were R 4.2.
I told them (and their company) not to come back to my premises again.
Another contractor completed the job.
Knowing even 10% about a particular trade helps evaluate the quality of the work being done.