Waterbed DIY Hands-Free Dildo?

I looked for a similar thread but couldn’t find it. Does anyone have suggestions for building your own handsfree anal dildo setup that works with a waterbed? The goal is to mount the device so it stays put, allowing your body weight to push it deeper in and out as you lift your hips. Perhaps mounting a dildo on a shelf or screwing it onto one, then filling the space with pillows…