what cone do you want to reach? you could do nothing & do saw dust,
pit fire style fires. go too high & the can melts.
Kaowool cloth insulation can be used, but it's not too cheap. get the
thinest koalwool, maybe 1 inch thick, and you can do some bisque fires.
if you smear the walls of th ecan with fluffy clay, a few inches thick,
you reduce your chamber volume but might get buy with something.
fluffy clay could be high temperature clay plus vermiculite or inert
materials that burn out like saw dust.
i priced vermiculite a few years back & it wasn't cheap.
You could line the kiln with a ceramic fiber blanket. the ones on ebay are
rated to 2300 degrees and are about 1 inch thick
i recently checked and they were under $20 dollars a roll. you will need to
make some ceramic buttons and buy some high temp wire to secure it to the
sides but if your lucky find some old electric kiln elements and cut the
wire to the length you need.