We all hate blast-back from flat blast deflectors. The CHAD remedy would be to add an overturned clay flower pot. Non-conductive and flame resistant.
A more refined approach would be to mold a ceramic blast deflector from beverage bottle ends and floor leveler compound. Sport Rocketry outlined the process in a back issue, and possibly the member's guide. I may post photos of examples in the near future.
That last is a nice idea. Why leveling compound? My thought would be clay, pressed against the mold into a shell. Or if poured, plaster of Paris or the like.