Seeing as the bottom third of your riser is firebrick, the metal liner of the top part may be fairly durable. Even so, I would not use loose vermiculite or perlite as insulation. I would use a perlite-clay or vermiculite-clay mix, with just
enough clay to hold the insulating particles together. I have not used vermiculite in this way so am not familiar with its mechanical properties, but I have heard that it is easy for vermiculite to crush and compact when packed in with moist clay, losing its insulation value. Perlite can work very well in this use. I would make perlite-clay at least 2" thick, vermiculite-clay perhaps a bit thicker. Someone should have a definitive suggestion for vermiculite here.
The proper way is the way that works best given your particular resources and needs
But if you have barrel rims to accommodate the lid-closing bands that removable-lidded barrels have, and the clamping band, that would probably give the easiest combination of secure fastening, smoke sealing, and accessibility for servicing. You would want to replace any rubber or plastic gaskets with woven fiberglass or other noncombustible yet flexible material. If you have welding tools and skills, you can mount brackets (probably four or so) to top and bottom to be tightened with bolts.
