I wanted to stay away from specific weights, but I agree that what I've got so far doesn't really work in this use case. Without weighing items, I think the best way to handle this would be to say some items use up more than one slot. Maybe a lead ingot takes three slots? That'd fit three in a backpack, which seems reasonable to me. To take this idea a little further it might make sense to impose a limit on the number of slots an item can take up in a single container. By that I mean the backpack might be able to only hold items that use one or two slots. The lead ingot is too dense would rip right through the canvas. Personally I think this is a little too tedious, but it might appeal to players who like bean counting more than I do.
So one thing I was considering was to also track how much access you had to items. That was one of the complaints in the link at the top of the post. The way I'd represent this is with a dotted line somewhere in the container. Items above that line are on top and easily accessible. Items below it require some rummaging to get to. I didn't include this option original because it takes place at a level of detail I don't usually play with. I'd like it in some games, but in D&D it would just encourage me to get a Handy Haversack that much sooner.
Anyway, as it relates here, I'd treat the pole and bundle both as large items, but the bundle would have no accessibility - everything in it would be below the dotted line so you'd have to rummage to get things out.
The big disadvantage I'm seeing for the whole system so far is that it's binary. You can carry or you can't. I'm not sure what sort of penalties to impose if you carry too much. (I suppose that might change if I picked a system so I could have penalties... The obvious idea to me is that you can carry additional large items and each one imposes some penalty or another.) It's more of a container/storage system than encumbrance system at this point.