I had a set of those once, they were useful when learning to ride strapless surfboard in the waves, helped to get out over the big whitewater but they were too "floppy" , I see you made a way to make them more rigid.
They are called NSI "Kling On", At $50 plus shipping I'd make a set before buying them again, they are just 3 layers of high density closed cell foam with a layer of thin plastic for the bolt head to sit against.
I think I'll go with two angled front loose footstraps for these boards.