Without seeing the stumps, I'm imagining them being a couple of feet high. ( If they are near ground level, stop reading). Before you cut a depression in the stump, could you just plant a large pot and anchor it, maybe with a length of rebar into the juniper?
I ask because the junipers produce an oil that inhibits other plants from closing in on its territory. I would think you would achieve the same effect, without all the effort it would take to hollow out the stumps.
