As for the external power supply. Most small devices come with an external power supply ours is just a big devices that needs one. We could make our device bigger and have the power supply inside which would mean a larger shipping box, and a larger package sent to you. That would mean extra shipping costs. It would also mean more engineering of the airflow inside the case. (all fans must point the same way really) This comes under the waste of time category a far quicker solution is to let the customer supply and use an external power supply, (like all of the smaller components) Then you can simply have it on the outside of the case so it wont get in the way of the airflow. Yes it fits on a 19inch rack shelf, There is space behind the unit for a power supply on the shelves (this is what we are doing in our hosting), You are welcome to use cable ties to make it look nice and keep it secure if you want to.
Another one of our internal rules is that unless its 100% essential we simply dont spend time on it.
Obviously you've considered rack density of the units? What are your projections? 6 machines per cabinet? The machine is ~4.5RU and shelve minimum of 1RU for anything stable. Getting more and/or larger power rails run to a rack is far cheaper than the square footage consumed by a cabinet in a typically data center contract. I merely mentioned zip ties before in jest regarding the rigged piles of FPGAs some folks had.