a 5g metal differential oil barrel from an auto shop, the ones with the tops that crimp on. Cut hole in top, slide in shop vac and duct tape it, make another hole for the sucker hose and go to town. Those buckets are pretty tough...who knew?
The larger the container the worse this might get. That "pressure per square inch" adds up pretty fast when you start increasing the surface area
Don't forget there are TWO ways of supporting a container. You can solder in ties internally, or construct something externally. Gallon paint cans solder really well, LOL
The other thing........are you certain you don't have a big restriction in the liquid line? I can't imagine that unless it's near freezing or below, that oil would be so thick as to cause this kind of grief