Well initially it's the weight more than anything. I want the whole lot supported on boards over my open top barrel so that if it does leak then the liquid only falls into the barrel it's above. The bigger it gets the more precarious I feel about balancing it

Doubly so when the bottom dishes out under pressure.
I'm glad I'm not the only one that's not so keen on screw tops. I stopped using them when one fell over in the car and the top popped off. Oil everywhere. The clamp tops however, I've dropped those from height full of oil and they just bounce. And I know from storing Meth in them on sunny days that they can take some pressure.
So perhaps a 30l clamp top is the way forward. Even if I don't have enough medium then 15l at the bottom is fine by me - just got to find one!