Too many steps. First you track down and buy a kit, then you have your rail milled, then you have your body milled. Each step adds time and money.
This next part I say with some reservation. As an indy game maker and an active member of those communities I often see people pop up and say "I have a great idea, will somebody (who has dedicated years and invested their own money to learn how) code it for me!?". It happens with whack-a-mole frequency. So I understand when I suggest this that the custom fabrication folks here on AO have their hands full with their own ideas careers and families already. But if anyone does have the time, and there is enough interest, I would love to see a built to purpose pump body/rail/kit.
I have a few thoughts about how this could be done while keeping cost down, but I'm sure the pro's know better than I. Would anyone else like to see this?