An update on my irrigation system using an elevated tank and clay cones, which are actually designed for PVC bottles: After a bit more searching, I found some inexpensive silicone stoppers that work better than the rubber ones because silicone is more flexible and doesn’t crumble. When buying the cones, make sure they’re well-made. The inner rim was very uneven in some places. Two of the cones broke apart when I pushed the stopper in. I then reinforced the rest with duct tape. It’s still not completely airtight, but that doesn’t matter. The bumblebees love the water source and come by regularly to drink!