How do you secure them?

Custom Metal Mounting Bracket for Adhesive Dashcam Lenses
Another custom solution from The Dashcam Store. Use our metal mounting bracket to securely place a second camera lens inside your car or truck. Made in USA

It’s a simple metal piece that has serration on the smaller/tab so that it holds in the space between the rear headliner. Then I just added some Crazy Glue (Harbor Freight version) between a flat magnet that I had from AMZ and the adhesive pad on the cam itself.
Including shipping, I think it was $23 which felt like a lot for a small piece of metal, but it solved my challenge instantly and people need to make money for their creativity and marketing.
Perfect height for rear cam footage.