Here is a simple CAD view of our bumper mounts. It is just 1/8" steel that is screwed to the bumpers with 5/8" wood screws. The holes are 1/4" and the ends have a hole in the middle that is drilled and tapped to 1/4-20. On the sides the holes are drilled through the frame and bolted using a 1/2" 1/4-20 bolt and lock nut.
We used velcro our first year of reversible bumpers, and it drove me nuts. Between the velcro not holding together or pulling through the stitching that held it to the fabric, I swore never again. Elastic is so fast and so easy, I can't see any better way of doing it anymore. I have heard the robopromo fabric pieces are nice, and they do save a few headaches too.