I don’t know of any portable devices and wonder if any exist.  Smoke detectors have recommended placement on ceilings near, but not next to, walls and I have trouble seeing how you could temporarily attach something that didn’t damage a surface.  I think CO isn’t as much of a problem as I believe it disperses throughout a space so placement isn’t as much of an issue.

How about a normal battery powered unit that you clip to an existing one with a bent paper clip or something?  Placement might not be optimum but it would at least be close (if the old unit was correctly mounted).  Don’t hang one from a water sprinkler; you don’t want to trigger a flood.

  I like the idea of this request though and will continue to follow hoping useful devices exist.

Note: ionization smoke detectors have a 10 year life expectancy.  If your devices are older than this they should be replaced.   The whole detector, not just the battery.

I have used this one in the past for the smoke detector (no CO):

First Alert Photoelectric Smoke Alarm

They are tiny and do not contain radiation, which I guess could be an issues to bring on planes.

I fix it with some removable poster strips.

The hardest part is not to forget it in the room when you leave… For that I fixed a long red ribbon to it so I don’t miss it when clearing the room.

I hope this helps.