For room grows the best reflective covering I have found is flat white paint. It is supposedly around 85% effective when compared to the reflective material now used in tents.
I even put white construction paper down under the pots. It saves your floors from minor spills and reflects light back up.
Before mylar we played with many things. Flat white gave the best results.
One covering I would suggest avoiding is aluminum foil or you may learn about hot spots rather quickly.
Day 25. First day of HPS @ 18/6
Four THC Bomb shorties surrounding my mutant. Tallest is 15"
View attachment 573028
80° F / 36% RH
1000W HPS @ 20"