I just finished setting up our bed presence sensor using this post as well and it's my favorite sensor so far. I can understand why no one sells bed presence sensors in a kit since it takes some tweaking to get it setup correctly depending on the bed mattress as they all weigh different amounts but the advantages of knowing everything is set as expected when you go to bed and when you get up and being able to change certain automations from timed to sync with you getting up is worth it.
The minute or 5 in the following are to prevent false positives if one or both of us get up in the middle of the night for a few minutes.
A minute after my wife gets in bed between 7pm and 6am, her reading light comes on and the bedroom lamps turn off if they were on. Then in 5 minutes if I'm traveling or after I come to bed the security system is armed, the bedroom shades are closed if they were open, the security cameras are put into night mode for alerts, the garage door gets closed if it was forgotten open, the locks are all checked and locked if the doors are closed or if one is open our Sonos in the room will alert us, and all of the lights in the house are turned off.
5 minutes after one of us gets up between 6am and 7pm the kitchen and family room lights are turned on, and the security system is disarmed. Then if the other is traveling or 5 minutes after we're both up the bedroom shades are opened, and using a virtual motion device with an Alexa routine, Alexa announces today's forecast.