Hello,

Not to sure how to go about this, and would appreciate any thoughts.

Whatever I do, I want to be sure that it is "Code" compliant, fully.

Have a quite old smoke detector on ceiling. Doesn't work reliably

It's AC house wired presently, but I wish to replace it with a regular
battery powered one without any 110 V house wiring going to it.

The AC wires going to it just exit the ceiling thru a small hole.
No metal wiring box like you always see; just a exit hole for the wires thru
the ceiling. Other things on the circuit, so I can't just disable the
wiring run at the service box.

Any new smoke detector I put up will be flush against the ceiling, and I am
pretty sure there wouldn't be enough room in it's base for me to terminate
the wires in, in place of a metal box.

I can of course put a wire-nut on the wire ends, tape it to be even more
sure, and push them back thru the hole.
But I imagine that this would not be Code compliant.
True ?

If I do have to go to a metal box, what kind do I try to find that would
enable me to just cut a somewhat larger hole and install it ?

There are no wooden ceiling joists or structure by the hole to fasten the
box to. So, the kind of boxes that have that adj. bar that attaches to two
ceiling joists does not seem appropriate.

Any new enlarged hole I cut I would, of course, like it to be minimal.

So, how would I anchor the typical metal octagonal wiring box ?

Is there perhaps another type that is made for the problem I have tried to
outline ? Sure would be great if so. If there is, who makes and what is it
called, please ?

Any thoughts on how to go about this would be much appreciated.

Thanks,
Bob