What was made from the bronze censers of those who died in Korah's rebellion?

Answer: An overlay for the altar

Eleazar had the censers hammered into sheets to overlay the altar, a reminder that only Aaron's descendants may offer incense.

“As for the censers of those who sinned at the cost of their own lives, hammer them into sheets to overlay the altar, for these were presented before the LORD, and so have become holy. They will serve as a sign to the Israelites.””— Numbers 16:38 (BSB)