This is my feeding mechanism for a homemade rival box mag. It isn't perfect, but I haven't had the time to tune it. Hopefully this will help advance the development of large-capacity rival blasters.
Various designs of the impeller have been provided with different motor shaft hole tolerances. Top and bottom gears without "rim" in their name are basic, plain impellers meant to be a starting point to build upon. "0-2" and "0-2-5" signify 0.2mm and 0.25mm hole tolerances respectively; however you may need to adjust the model and/or your print settings for good fittings. Impellers with "rim" in their filename are the final design I used in my demonstration video.
The motor used is a geared, high torque 12v motor by UXCell here: https://www.amazon.com/dp/B01N990WOU/ref=twister_B074J2T8VK