Boarding step insert for '59 Glastron Fireflite

After 55 years the rubber inserts for the boarding steps on my powerboat are severely perished, shrunken and hardened. I tried to take a mold from one of the old ones several years ago when I first restored the boat, but couldn't get a decent impression. Now that I have the capability to print one, I'm trying again with a computer-generated mold.
I plan to fill the mold with this modified silicone: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&sqi=2&ved=0CB0QF