Soapstone is an ideal material for thermal mass, but is not an ideal insulator. What you want around your riser is an excellent insulator that can stand very high temperatures. The ceramic fiber is that already.
You can perfectly well make an enclosure of brick instead of a steel barrel; the top third or so, at least,
should be firebrick so it will last under the heat stress. This can be faced with a solid soapstone layer for additional mass and good radiating properties. You will not get the same instant heating as you get with a bare steel barrel. I would always build in an access panel to be able to inspect and repair the riser if needed, and if well located, this access panel could serve some instant radiator function.