Satanite refractory is the usual go-to for forge refractory lining. After that has cured, you can apply a product called 100HT (also called ITC-100) to the insider to help boost the speed of your forge's heat time. It's a ceramic coating that works as an infrared reflector, concentrating the heat more in the center of the forge.

Refractory cement can be found here:
https://www.amazon.com/s?k=satanite&crid=JM9LGT54Z2K&sprefix=satanite%2Caps%2C185&ref=nb_sb_noss_1

ITC-100 can be found here:
https://www.amazon.com/dp/B096RNM3CQ?ref=nb\_sb\_ss\_w\_as-reorder-t1\_ypp\_rep\_k5\_1\_6&amp=&crid=Z1MDD8NSG2OH&amp=&sprefix=itc100

Probably want to use a refectory mortar with a ceramic reflective coating over it.

This is what I would use

https://www.blacksmithsdepot.com/catalog/product/view/id/18919/s/satanite-refractory-mortar-5-lb/

With a coating of this

https://www.amazon.com/dp/B096RNM3CQ/

Just did this a few weeks ago for my double burner 16kg furnace. Will add progress pics and all the details after work today for ya. But used the following with plenty to spare.

https://www.amazon.com/dp/B096RNM3CQ?psc=1&ref=ppx_yo2ov_dt_b_product_details

https://www.amazon.com/dp/B08S5SY61X?psc=1&ref=ppx_yo2ov_dt_b_product_details

https://www.amazon.com/dp/B095XJ1BDB?psc=1&ref=ppx_yo2ov_dt_b_product_details