The insert is a good idea but I would rather use that money to buy other stuff for my car.
The concept is good. I would simply buy a new lap belt. Found a complete set at a local tuning store at 30$.
Use a piece of foam and wrap it in black vinyl with glue between vinyl and foam.
Easy job. Total cost less than 50$.
I would store the insert in the spare wheel area and only use it when needed. The belt could always be installed and hidden under the seat. I love the rear seat as is. It gives a luxury look to the car.