Of all the 9mm speed-loader clones on this site, I thought C4Matt's speed loader was clever and effective -- elegant, even -- but after loading a few mags, a bit unrefined.

Changes?

• Shrunk the outer walls a bit (smaller, but still plenty of meat to survive the abuse).
• Added finger holds to the rear (smaller, so something to hold on to)
• Added side-guides and angled back-stop on the rear to funnel the mag into the slot
• Created a short step to trap the cartridge against the front of the slot (drop the cartridge in from the rear and it catches on the step as it bounces off the end)
• Created slots for the mag's lips to slip under the trapped cartridge (step only traps the cartridge, not the mag)
• Reduced the slot length so the mag lands on the right spot on the bullet being loaded to push the stack in evenly (too long and the cartridge catches on the previous bullet instead of the previous casing, too short and it catches on the narrowed lips)
• Created shallow ramp above the front slot for the mag lips to clear enough to fully seat the bullet (otherwise the front of the mag catches on the semi-circle and the cartridge has 1/8" left to go. With the ramp, the mag hits the stop on the nose of the now fully seated cartridge)

Workflow for loading is easy-peasy:

• get a good grip on the mag with one hand
• With the other hand drop a cartridge in the slot from the rear
• grab the finger holds and lift the rear ~1/2", and the cartridge will slide forward over the step
• punch the mag down from the rear of the slot, then slide forward until it stops
• lift out, pick up another cartridge with the loader hand and repeat another 9-12 times (yeah, small mags :( ).

Bulkier, more complicated loaders may be a touch faster, but try it out -- you can build a surprisingly fast rhythm in a mag or two with this tiny thing. All you need is a flat surface stout enough to resist the mag's springs.

While this is tuned to the M&P Shield's 9mm mags, it likely will work for any 9mm single-stack or a double-stack with a long (~15mm) single feed funnel in the body.

Creality

Ender 4

No

No

Keep your walls to at least 3 perimeters, and give your top/bottom layer count high enough so it doesn't switch to infill in the bottom of the slot (5 top/5bottom at 0.2 mm layer height) that's the area that will take the worst abuse. Early prototypes where the bottom height wasn't enough showed some denting, but later versions were much more resilient.

That being said, skipping all this tuning and just going 100% infill won't cost that much more time/filament.

I used PETG almost more for the thermal resistance (tucked in a black bag in a hot car on a summer range day, PLA might get a bit soft), but YMMV.