Compatible with vase mode!
I remixed my Low-poly Rose Twist Vase into a magnet so you can put it on a fridge, whiteboard, etc. Yes it should hold water just fine (depending on your print settings) and a nice little flower or two!
There's two slots in the back for 8x3mm magnets (cylindrical or not). Simply insert the magnets into the bottom and slide them upwards where they should get "snugly stuck" (the top is smaller on the outside-facing edges than the bottom). If the magnets aren't being held strong enough just use some CA glue--they'll stay put.
Note: I've included the .blend (Blender) file in case you want to scale it up or down and still have the magnet slots be the correct size (or add more/mess with it). Enjoy!
Update 20170925: I modified the model slightly so that instead of being perfectly cut in half (90°) it's cut at an angle to provide more room at the top for a bigger stem/object (ex: You can now fit a Sharpie inside). I also made the logo indentation much less deep so as to make leaks less likely as a result of backlash and/or the bleeding edges problem. I named the new model "2.0" and left the old one in case some folks prefer a crisp 90° slice over a more practical 95°.