Makeshift carboy cap for secondary..NEED HELP!

SO...

I transferred my brew to a secondary today and am prepping to make something tonight when I realized that I only have one carboy cap. So, I ran to the local brew store (took an hour because of traffic) and got another cap and an airlock. Simple, right?

I got home and realized that the new cap doesn't fit my carboy, although the idiot at the store clearly told me it would fit my 5 gallon carboy. So, I can make my brew tonight, but, I am short one cap.

Here is what I'm thinking about doing until I can get back to the brew store to get a new cap...

Since there appears to be no more gases trying to escape from my secondary carboy...(I haven't noticed any bubbles in the airlock for two days)
I was thinking I could just cap off the top of the secondary with aluminum foil and tighten to the carboy with one of my wife's hair ties. That way it hopefully holds in all the good gases, and since it doesn't seem to be doing any more fermenting it will hopefully stay put and not blow off in the next 16 hours.

Then tomorrow I could run back to the brew store to get THE RIGHT CAP and then place it on my secondary.

Then, I can brew tonight and get my next beer going...

Is this kosher to have the secondary set up this way for up 16-20 hours?

Saran wrap sticks well and would probably last fine overnight. Just keep in mind that even though it's not bubbling when you watch it, doesn't mean it isn't at all. If it changes temp at all, it will either suck in air, or blow it back out.

Overnight in a room that doesn't have a ton of airflow stirring up particles to get inside though, I'd think you'd be fine.

Is the stopper too big or too small? If it's too small you can use some black electrical tape, wrap it around, until its the correct diameter. Then, sanitize and your good to go!

Now, if its too big, you may be SOL.

saran wrap with a clean rubber band, loosely on there, is fine.

M_C

saran wrap or tinfoil will work, just make sure its sanitized