Answers

Good Morning,

Nice space.  Its hard to tell from the photos just how big the space needing to be filled is however 

Dap 18854 High Heat Mortar Raw Building Material, Black

should work well. It can likey be found at you local hardware store of online

https://www.amazon.com/18854-Mortar-Building-Material-Black/dp/B0006MXS4C/ref=asc_df_B0006MXS4C/?tag=hyprod-20&linkCode=df0&hvadid=167127061369&hvpos=1o5&hvnetw=g&hvrand=16226564078373131268&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032160&hvtargid=pla-307130896304&psc=1

Best of luck and Happy Holidays

Jared