Think that, depending on the standard you want to finish it, it would pay to replace the heavily rusted top plate and get it sandblasted.
Probably overkill for a shed heater though.
Yes I have used vermiculite mixed with high alumina cement fondue to back fill chimneys around a stainless liner ...and it is very messy ...and takes a while to go off ... Have you looked into recycled kiln shelves ?...some potters replace them when they get a bit bent and that would not really bother you ...The ones I used for a large fire back I was building 20 years ago were about 35mm thick and 500X500, I think that fire is still in use..
firebrick usage for me screwdriver
i use the perlight and fire cement mix myself on stoves
but then again id cut that up and change it but thats me in doing so would make it better
which i will be doing soon on a new stove
The free heat feeling warmer is the same as free beer tasting better!