I filled any gaps between drum and brick or clay plot and drum with Perlite. Some use sand, but if there is a crack somewhere in the future the sand might leak into the main cooking chamber and you lose some insulation. Plus sand would make this extremely heavy project even heavier.
The clay part can't be built with just any clay. In India, clay from a particular part of the country is usually used to make the liners. Plus the clay must have the right kind of grog (sometimes hay, horsehair or other stuff) to give it some tolerance. Otherwise the expansion and contraction of repeated heating and cooling will likely crack an improperly fabricated clay liner. That's why I decided to go with the clay pot! It was getting too complicated...
Have a good weekend all! Heading to frigid Michigan for the weekend... hope to see some beautiful pron (and learn how to do it right) when I get back. Final assembly of the UDS when I get back.