A neighbor of mine was practicing dry firing with his 9MM and was previously using a bobby pin to keep the action slightly open. However the bobby pin kept catching on his holster every time he would draw or stow his weapon during practice sessions. Without the bobby pin he couldn't practice pulling back on the trigger and dry firing so he either had a clumsy stow/removal or he couldn't dry fire.
He reached out to me to see if I could hook him up with something so I made this after taking some measurements on his weapon and modeled it from scratch in Fusion 360. Since it had a simple function I took a simple approach, adding a little extra traction and visual flair with the pic rail like design. I modeled it to give enough strength when in the action, and still fit within the recess where the cartridges eject. It is pretty small, I printed it using PLA, but I would recommend stronger more shock resistant filaments when possible.
As always BE CAREFUL when dry-firing and make sure you are following all recommended safety precautions. This is not a safety device and does not replace or guarantee safety when using it. If you aren't sure how to safely handle a firearm, then DO NOT USE THIS MODEL, seek professional education on the subject. Last thing I want to hear is someone 'dry-firing' a bullet left in the barrel.
Use stronger filaments for better durability
Autodesk Rendering of the STL