How to temporarily seal off ductwork?

[BloomingtonMike]

I'm plumbing in my Dust Gorilla in my new shop, using Oneida ductwork. i have a few pieces on order. I think I can get enough machines hooked up that I can use them, but that will leave at lest one 5" line wide open to the room. Aside from putting a blast gate on it, what would be a good way to temporarily seal it off so that I can run my DC?

Thanks.

Is oneida ductwork snaplock, or spiral, or QF/nordfab? Norfab makes endcaps that snap on and spiral pipe you can get endcaps in the stove pipe section of menards and prob any box store. Or like was mentioned, get a circle cut with a lip and make a cap.

[Cooler]

I would use a latex party balloon.  I would simply blow it us while it is in the duct.  The life expectancy might not be great, but it is cheap, fast and effective.

I have used balloons to pack delicate products for shipping.  They always seem to make the trip with no problems. 

You can get balloons almost anywhere.  I would get one that can be blown up to be quite large.  It will be made from thicker latex.  

Walmart sells 12" diameter balloons for $7.00 online for 72 pieces.  I'm sure you can get smaller packages in the store.