dont use aluminium foil! it focuses light into hot spots and cant roast your plants! i use white printer paper or white gloss paint, NEVER USE FOIL...! thats the no.1 rule, people confuse MYLAR with foil, dont worry, its a mistake thats easily made, just rip down that foil asap,

p.s. I like the shower idea, i would probably only use it for seedlings and adolescent plants, then move adults somewhere more airy