I realize that my suggestion might be more expensive than most people would want to deal with - but - I wanted to be able to offer very high quality viewing on a remote monitor so I picked up a couple of field monitors which can be powered all day with a large Anton-Bauer field battery (Lilliput 28" & Atomos 19"). Of course that meant buying the battery, which was almost $3,000 alone.
For more versatility I added the Vaxis 1000ft 10-channel wireless transmission system. With that system one can park the remote monitor(s) as far away as need be without a long cord - a block away if necessary. Of course, wireless is not necessary just to get your clients out of your oxygen supply. A long cord out of your controller's HDMI OUT will certainly suffice.
Although mine are high-end, expensive options they're sort of necessary since I anticipate working with a few law enforcement agencies down the road. At that point the quality and reliability needs to be very high. If it's overkill, in the meantime, I can always use it to blow future clients' socks off.