Here are the measurements you asked for. While not critical and you could certainly knock out something equally effective on your own, I know this one works. The rubber I used is about 7 or 8mm thick (1/4 inch or so). It’s reinforced with fiber belting. Get yourself some very sharp and stout punches - this stuff is incredibly tough.
The oblong holes allow the double length of cord to pass through, one above the other, without requiring the opening to be so large that the knot would easily pull through.
A word on bungee or shock cord - it comes in different tension strengths. the el cheapo stuff is generally a lot more “stretchy” diameter for diameter, and thererfore requires that it be stretched a lot more in order to generate much tension. This greatly taxes the cord and it weakens more quickly than the better, stronger stuff.
If you’ll buy the good stuff, 1/4 inch diameter (7mm) that is very difficult to stretch, you’ll find that you don’t need to stretch it more than just a little to hook the yoke on the Ama pin and even without much stretch the shock cord will hold the Ama securely, without bounce, in some pretty darn rough water. I think I cut my sections 16 inches long and knotted them as close to the ends as possible. I only have to stretch them about 1-1/4 inches to hook them on the Ama so there’s not much tension on anything yet they don’t allow the Amas in any conditions I've been in so far. I can push them down with my hand, but so far, no jostling on the water has caused them to move at all.