Tom Gordon (GordonInnovations) has a darn good design in his "Croc Heel Snap Replacement" (https://www.thingiverse.com/thing:1013692). It's tough (significantly stronger than the original equipment), visually matches the original equipment well, and is comfortable enough that you don't notice that it is there.. It is a well designed thing. I have just made a couple of minor modifications based on my experience with his original design.
Since finding his design, I have replaced TWO of the rivets on my Crocs. On the first repair (Tom's version), I had trouble getting the male part to fit to the female part. It took me so long that the super glue (cyanoacrylate glue) set before I had it fitted properly. So, I tweaked Tom's part by adding a slight bevel on the female part to aid the insertion of the male part (my version 2). It worked well, and away I went. I spent a few weeks with that repair and I had the back of the rivet pull through the shoe and detach the strap. So, I went back to Tom's design and enlarged the size of the rivet head (my version 3).
THEN I had one of the other rivets break. As a result, I have installed two of these modified rivets. They seem to work. So, I am sharing the V3 design with you. Tom did all of the heavy lifting in this design. I just added a bevel and gave a rivet a bigger head.
Thanks, Tom. Nice work.
When assembling, I found that I needed some sort of clamp to assist in mating the two parts. A one handed bar clam works, as does a C-clamp. I have used bot superglue and ABS glue to attach the two parts of the rivet. Both glues seem equally up to this task.
I used black ABS for strength and to match the original part.