lightproof question

I know there are a ton of these threads but most deal with preventing light leaks on the OUTSIDE of a box.

I hear 2 90degree bends are enough to block out light from escaping. Now in terms of making a stealth box, that makes sense because you have a lot of light in one box so its going to need several bends to stop ALL that light.

For my box i just want to prevent ambient light found outside my box from getting in during flower.

To do this would one 90degree elbow (painted black inside) be sufficient to keep out outside light from getting in? By outside light i mean literally one window and 2 shitty little cfl lights that light my room (my rooms pretty dark as it is).

This box doesnt need to be stealth as it is a legal grow, im just keeping it small 1 plant with cfls and i have 1 pc fan for exhaust that i need to light proof and 2 small exhaust holes cut along the bottom that need light proofing.

I wanna use those big black 90degree pvc pipe elbows they make since they are cheap.