Check my thread in my sig to see what I did for a light trap. I do reccomend using ducting without reflective surfaces in the middle though as it releases some light from the cab. I will be perfecting that.
I would also check out Dark Room Louvers as they can be used as a light trap for your intakes.
For exhaust, you shouldnt have light if you are going to use ducting and a scrubber. There are many DIY mini carbon scrubbers for the type of application are you asking about. If you are to purchase one, I recommend a small 4" flanged filter and you can get them about 8" long. Dont cheap out on filtration. It may seem like a lot of money to start but if you want a stealth grow, it is necessary to have all that dank air scrubbed out and cleaned before releasing back into your room.