Panda plastic is excellent tho I've found that it isn't 100% light-proof so I use a double thickness in critical areas. For flat surfaces you can't beat Diamond Reflective Sheeting- to quote-" Diamond Reflective Sheeting is 'diamond pressed' which increases light diffusion in the grow room meaning no more hotspots! It is also easier to handle and more resistant to creasing than other types of reflective sheeting on the market. It protect's against heat and moisture with EMP (Encapsulated Metal Protection) technology and gives an even light distribution due to the diamond pattern.
100% Lightproof
50% more reflective than standard black & white sheeting
Flame Retardant with Total Blackout". It's a lot stiffer than mylar & virtually crease-proof. The plastic covering makes it a doddle to clean- I use it for my growroom (wardrobe) floor. Three times the price of ordinary mylar so a bit costly for large areas tho.