I checked Thingiverse for seat belt slides, but could not find one that was easy to adjust. I decided to design my own. This one has 3 parts to be assembled. It uses sponge window foam to function like a contraction spring. Works perfectly (smooth) and looks great.

This design uses a press fitting so it can be assemble around the seat belt in the car. I used MD Sponge window seal 3/8" thick and 3/4" wide 10 feet is about $4.00 US. (see photo of package) Actually you only need to cut a 50 mm long section for each clip. The foam has adhesive on one side so it can be adhered to the clip before it assembled. The last plate has smaller slots and is a simple press fit onto the two leg spacers. Once assembled around the seat belt, you can simply press the last plate onto the two legs. If you need a tighter squeeze on the belt, you can use a pair of channel lock pliers to press the end of each leg into the final outside piece a bit further. This design uses 8 mm gap for about 9mm of foam thus it always has a small amount of pressure on the belt, so simple press fit should work fine.

UPDATE - by doing a bit of research I discovered something that is standard on most rear car seat belts. Most car makers have equipped the back seat belt mechanism with two modes. One for the installation of a child seat. For a child seat to be held snuggly in place you pull the seat belt completely to the end, and then let it retract in. When it retracts you will hear it click. Any slack can be removed by letting the belt retract, but once it retracts in, you cannot pull it back out. (This helps retain the child seat snuggly in place.) But if you accidentally pull your seat belt out all the way in order to fasten it, you will activate this child seat mode. If you do, just let it retract all the way in, and pull it out again, just do not pull out too much. then you will not need this belt retainer.