These photos show the priocess I used, however I got the size wrong, so I ended up doing the whole process three more times before I got it right. So all these photos show an under sized strap, but the process is the same.

I traced the shape on to some MDF

Like I did on an earlier piece, I used strips of self adhesive weather strip for the ribbed pieces 

Then I cut a stack of 6 MDF end tags 

Then I glued them in to place and boxed it up

A coat of petroleum jelly to stop the silicone weather strip from adhering to the casting silicone

Silicone mould 

Positive silicone 

Some Ultracal 30

Latex poured in.

A spray of Plasti-Dip and they are done! Then I just had to go back and make them the right size, but the photos show the process anyway.