My original parts were re-worked off the vacuum formed parts I did back in 2012.
Once I molded and cast these, I made several sets which have all been used or sold. The parts were nice, but a touch too round, so I reworked them again.
http://www.therpf.com/attachment.php?attachmentid=644972&d=1468716241
This is my latest version. I have not really cast any parts yet. The original part is on the right. The dimples in the new part look overly pronounced, because of shadows.
http://www.therpf.com/attachment.php?attachmentid=644971&d=1468714744
So what I did was to place half of the original mold into the mold box (butter box in the picture), flood it with D65 plastic and then resculpt. I then trimmed a bit off the bulge and added the dimples to look closer to the screen used shoe buckles.
After I was happy with the new master part, I placed it back into the butter box and fill the container with silicon. I repeated the steps until I had 2 x the 2 part molds.
The first test I did used the same A65 rubber I use for the soles and the strap tabs seemed too weak. It might be possible to pour the body in A65 and then reinforce the tabs with A100.
The buckles currently on my shoes are all A100 and whist they don't flex, add real support to the back of the leg and are very comfortable.