problem with grow room

I'm having a hard time setting up my room. I'm doing it in a crawlspace in my garage. Trouble is the crawlspace is underneath my living room stairs, and there are small cracks in the stairs so light comes through. What can I use to cover up the light on the back of the stairs? I've tried putting up thick, black garbage bags but the light still shines through.

i would fill the cracks with some wood putty or the foam in a can where you can spray it in the cracks.

you could also put up ply wood or something solid and seal the edges.

foil tape