Plastic bags and caps work fine for me for deep conditioning. Saran wrap works like this cap for baggying.
The bold in this part is why I don't think I would be happy with this:
I do not like conditioners or products getting on my scalp, let alone into my scalp. I prefer to allow sebum to flow out of it and condition my scalp instead. And when I apply conditioner, I avoid getting close to my scalp and usually leave my hair hanging down so the conditioner can drip to the ends where it's needed most than towards my scalp. When I put on a cap for deep conditioner, I like it to be loose so my hair isn't pressed against my scalp. I don't believe I need to press conditioner into my hair with a cap. Besides, I prefer the "steaming effect" of the looser cap. And I don't get drips because if using a plastic cap, I wrap Saran wrap around the perimeter to ensure no chance of that happening. If using a regular plastic bag from the grocery store, I create a vacuum suction effect around the perimeter by making it stretch a little so there's no space.