Can I fill this crack?

[bulldogc]

I'got a heatilator ec42 with some panel damage on the back wall. Someone is coming out next month to replace the panels but I am wondering if I can do a patch with some morter as a temp fix for the holidays. Probably 2? Or so fires unless we loose power or something. Wood fires only.
Couple of photos attached. Crack for scale is nickel width in a couple of places somewhat smaller in most of the rest. Unclear how deep though.

 

[DAKSY]

Check the depth. If it goes through to the rear wall, don’t burn. If it doesn’t go all the way through, go ahead. For a quick fix, RTV Silicone will work, but it won’t look very nice. Another option would be to place a cast iron fireback in there.