[it wasn't me]

I want to blow a gate off its hinges.now I know you can shape plastic explosives into pyramids and such to focus the energy on one point but my problem is that I'm not gonna be using c4 I'm gonna be using black powder.I've a fair amount of experience making `fireworks` :-D and know that when the powder explodes inside a container the blast escapes thru the containers weakest point,naturally,so I'm looking for a container that can 1. hold a fair amount of black powder 2.is solid enough to not blow apart in all directions (as I want to channel the blast towards weakest point) and 3.has a weakest point which I can direct at the gate,so the 2 fireworks mounted on the gate behind both hinges detonate and the blast energy is focused forward against the hinges blowing the gate off.I may use another two on the opposite side for good measure,blowing all four simultaneously.
Has anyone got any idears for such a container,existing or otherwise??? I know I could just kick the gate off its hinges but the point is not to gain entry,I just wanna to blow the gate off :-)