Stuck in outer-chimney building phase

Hello Permies, i need your advice! I've been working through my first RMH build. I'm hung up trying to insulate around the brick inner chimney. I read about using an electric heater's shell w/ perlite-cobb inside. When my sister got a new heater I took the old one and hack-sawed off a piece to fit the height of the chimney.



my concerns:
- Usable material? There are some rust-patches in an area of the panel. There is one hole that goes through, almost the diameter of a penny. I'm worried about the insulation holding vs breaking through over time.
- Access Panel hole. Due to chimney height I had to cut a piece containing the access panel for the electric heater. There is a low profile steel cover held by 2 screws that I can reattach. Again concerned about this holding the insulation in.
- Reattaching cylinder? After un-crimping where the edges come together I can't seem to get the cylinder to mate together again. Any suggestions? Should I use wire wraps and rivets to hold it in place as best I can?
- Perlite-Cobb - Is there any special clay I need to because of proximity to heat? I'm still searching forums for a good recipe

The insulation won't go through if you use clay and perlite.

Have you tried ratchet straps to close the steel around your riser?

Put the cover over the hole and wrap tape, masking or duct, around the exterior mold piece to seal it up.

If you want to make life easier you can research making your own castable refractory cement, and make an insulated heat riser like this.