The 300 is for a printer bed 300mm X 300mm X 400mm, like you find on Creality's CR10 V2 and V3. I use this size for rifle case feeders.
The Mongo requires a printer with a 350mm square bed like the Ender 5 Plus TylerR uses. I can't print that one on my CR10 V2.
The original normal TylerR collator can be printed on a Creality Ender 3 with a 220mm square bed. I prefer that size for all bullet collators and works fine for pistol case feeders too.
Below is a picture of the 300 (left) and the Normal size (right) that I have printed and now use. I only use Proximity switches with variable speed on/off speed control to control collator feeding. As per TylerR's suggestion....I also use a relay to preserve the prox. switch. The big knobs in the middle cover the wonderful TylerR clutch system.....no more broken parts from feeding snafus.
https://i.postimg.cc/Hn12w0QX/IMG_4005.jpg