My 50c solution may not work for you - it probably won't fit in your holder - but it works well for my handheld pointer and might work for you in a different kind of holder.
I used a small hose clamp that fits loosely over the button. If you only do up a hose clamp loosely, it's natural shape is more egg-shaped than round - it can be adjusted so the smaller diameter presses the button down while the larger diameter doesn't. I use small plastic cable ties as 'guides' so that the hose clamp doesn't slip out of place. Oriented the correct way for left- or right-handed use, the hose clamp is easy to flick with a thumb in the dark and stays where flicked - 'on' or 'off'. It's surprisingly reliable and easy to use. There is a small risk of it turning on in a pocket but that's only happened once.
See photos - only links to photos because thumbnails never work for me except in preview:
https://ibb.co/mdY1d2B
https://ibb.co/NnnjL09X
https://ibb.co/CsbrhP2s
Actually, I can't recall when I last used my pointer - must have been several years ago.
This solution from 2008 might also work:
https://www.iceinspace.com.au/63-469-0-0-1-0.html