I'm on the same thoughts as you snowdog,
There must be something at the garden centre that we can buy 2 off, then up turn one of them, and then cut a side hole in the lower one for the air, and a hole in the centre of the top one for the food entrance.
Now that makes sense to me, and if your worried about it exploding when it comes into contact with water, then im sure there are many paints you could apply to the outside of the pot that could resist the extreme tempeture.
Just my 2 pence worth.