no lid is fine. make sure your temperature isnt too hot for the molly.
plus, i've had bad experience with V traps. i put my guppy in, she just wont bear the young, i take her into a net box (another kind of breeder trap), put some plants, a few days later i got 20 or so fry.
agreeing with trickster about stress on fish. V traps are way to small. V TANKS are good, but internal tank V traps ive just had bad experience with.
my 2 cent