How do I stop my in-cabinet lights from falling out?
I have similar looking lights in my cabinets. The tabs on the side are meant to engage a plastic retaining collar mounted to the cabinet. The previous owners did not install the collars, but they did leave them behind in the back of one of the cabinets. Unfortunately, there were 3 missing.

For those three, I used a staple gun to make a little lip just inside the edge of the hole. Here's a picture of where I would put the staple(s) in your cabinet.



My lights had 4 tabs, so I used 4 staples in the proper position. I also used a screwdriver to back the staples out a bit for a better fit. The first one I did I just pried the staple out a bit. But for the subsequent ones I held the screwdriver against the edge of the hole and just stapled over the top.

Then to install the lights, make sure the tabs and the staples are NOT aligned, insert the light, and give it a little twist to slide the tabs over the staples.