Yes, there is more than one way to heat treat stone.
I insulate the inside of the lid with fiberglass and cover it with heavy duty aluminum foil. I wrap the foil up and over the rim of the lid and it keeps the insulation in place. This interferes with the seal of the lid, so I drape a canvas cloth over the oven to keep out drafts. I use plain old fiberglass insulation for the lid. Because of the out-gassing issue, I used to keep the oven outdoors but the moisture (dew and rain) causes shorts and burn-outs in the control wiring (behind the temperature dial). I now keep the oven inside the ventilated area of my shop.
I don't worry how much stone goes into the roaster but I usually fill it at least 1/3 full. I've heated single large chunks before without any issues.
The guys that I know that have the fancy, expensive kilns never use sand so I've never used it either.