NO LIGHT! that promotes algae growth and you don't want to deal with that. Alum foil blocks out light but it tends to trap heat. If you have money to spend go get some black and white polyurethane. Leave the white side up to reflect light and you won't have to worry bout any light getting through
or you could do wat i did...put them in black grow bags...very cheap theyer abiout 50cents....and the smaller ones are about 25cents...... blocks out light and works fine..
Yeah, like everyone said you need to either re-pot your plants, or use something to block out the light from getting to roots or else you asking for trouble.