I have tried using a centre vortex slot in a J tube before with OK results but a single vortex seemed to work much better in the J tube, however, with this design a piece of ceramic glass above the burn chamber allows very hot air to feed directly into the base of the riser.
My first bit of advice is that if you are going to be a mime, you shouldn't talk. Even the tiny ad is nodding:
It's like being on a deserted island - only with gardens and natural buildings in Montana