Here's my self designed bass mute:
It's made from two pieces of steel I've sawn out of a harmonica holder. Further some screws and springs and a piece of foam.
It actually could also be glued to the body with 2-sided adhesive tape. It works really well, but has to be custom made on the bass.
So in your design: Don't forget to imitate the curve of the strings in the piece of metal that holds the foam.
There is usually very little room underneath the strings to successfully contain the two pieces of metal and the foam in the 'resting' position.
That's why I believe the Elio Martina Bass Mute has the foam blocks on the side of the strings. It's a very good design that leaves very little room for improvement.
But hey, mine is waaaaay cheaper!