vacuum container??

I picked up a vacuum pump this morning to siphon the oil out of transmission.
I tried using a very thick plastic jug to catch the fluid and the jug just buckled.
I tried a 5 gallon bucket and that also buckled. I need some ideas what to us to collect the ATF fluid without it buckling.
I have a new empty paint can but that will not work. I am open to suggestions.

I may put in reinforcements into the bucket to hold open the sides