How can i create an object to fit another object with an angled extrusion?

Hi guys,

I've got two object one which is a container with a hole at the bottom with angled walls and I need to create another object wich will fit in there with a margine to stop it from sliding through. The problem is that that I don't know how to create that margine in an angled way. This is how i did it in a normal way if the surface woul be straight, My angle is 19 degrees so maybe i need to create a cut in that margine all around with that degree? I created a plane to cut it but how can i go all the way around? Many thanks. Hope it makes sense what im trying to explain

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