crumbs there is a finite amount of available space to make a rackmount unit.  You design a rackmount unit with better airflow, buy modules from HF in bulk and then resell the package.  I think you will find it is harder than it looks.   Putting radiator in the back would be an "easy" solution except you only have 17" by 1.75 x U height inches to work with.  If the power supply is mounted on the back and the radiator is mounted on the back the radiator will be tiny, too little surface area to effectively cool 750W+.    To keep Delta T less than 10C over ambient you are going to need 1 to 2 cm2 of radiator surface area per watt (i.e 420cm x 120cm on 750W heat load) even with pretty high extreme airflow (3000 RPM pusher & puller fans).  There is only so much surface area on the back or front panel of a rackmount unit.

Sure if you don't want to compromise then build a massively expensive 6U chassis with straight flow power supplies and the entire rest of the back panel devoted to a radiator.  Of course when you do so you would price yourself out of the market and people will just buy the more economical solution from Hashfast or Cointerra.