Described below is another way of utilizing the thumb-latch of a deadbolt to determine the locks state (extended or retracted). With sufficiently tough plastic, this approach could be 3 D printed. One could also obtain some scrap plastic from a big box store. The version below is made from a short piece of scrap 3/4" wide steel banding obtained from a Loew’s store’s trash barrel (used to bind lumber for shipping). This version is compatible with Kwikset, Gate House, RB, and Schlage deadbolt locks.
One end of the banding was drilled, cut, and filed into a yoke shape that snuggly gripped the stem of the thumb-latch. See picture 1 above.
Since the thumb -latch on my lock had a void on the underside, I decided to use a small set screw as an anchor pin. This is optional. Probably some thumb-latches are not hollow. Anchoring the pin was accomplished by drilling and tapping a hole in the banding to engage the hollow space under the end of the thumb-latch. The screw was then Crazy Glued in the threaded hole to make it more secure.
The 15mm Ø neodymium magnet was Crazy Glued to the rounded end of banding. A penny was used to trace the round pattern for shaping. In this application, the banding was 2.5" long. Measurements are based on the size of the thumb-latch. See picture 2 above.
Here is a piece of scrap plastic banding shaped to fit the thumb-screw. See picture 2a above.
The device was trial fitted to the thumb-latch and found to need some yoke and pin filing. It was then Crazy Glued permanently. See picture 3 above.
This is a side view of the thumb-latch extension. See picture 4 above.
The Crazy Glue in the case of the steel banding approach just needs to hold the thumb-latch and the extension together. The yoke and anchor pin provide a mechanical resistance-couple independent of the glue. This pairing of latch and extension would work without the anchor pin because of the strength of the glue.
I calculated the tensile resistance of the glue contact area is around 300 pounds. I don’t know how this translates into a torque resistance force but in normal use, it won’t fall apart. If you wanted to remove the extension Heat the Crazy Glue joint to a temperature of about 150° C, the glue will soften. DON’T breathe in the fumes, IT IS CYANIDE.