I’ve arthritis in my knees since my mid-twenties, so I can relate to your plight, @Milo Farnsworth. Here are a few suggestions that have worked for me:
A couple of memory foam bath mats. I call them “coxksucking mats.” A comfort gel kitchen mat A bath stool (sometimes called a “spa stool”) up against the bed. Pro tip: the plastic ones ($29.99 @ Target and Bed, Beth, and Beyond - put those coupons to good use) also work for water sports ‘cuz you can hose ‘me off after you get hosed down
[*]Lay on the bed, head propped up on the headboard with pillows, him strattling you