To tighten a toilet seat with no access underside, you'll need to use straps or bands.
• Measure the circumference of your toilet bowl.
• Buy adjustable straps that are long enough to go around the bowl and attach them underneath the seat rim on both sides.
• Adjust the straps until they are tight and secure them in place with screws.
This method is an effective way to replace an old or missing bolt from underneath the toilet seat, allowing you to securely tighten it without having any access from below.