One thing I forgot to mention. There's a book with a title something like
'How to cast small metal and rubber parts' aimed at the classic car
restoration fraternity. It covers using this Devcon 90 product, and it
does mention that while it's not recomended (by the manufacturers), you
can weigh out part of the elastomer kit and mix it. The
unmixed-but-opened kit has a reasonable shelf life (month or so), so you
can have another go using it.
It might be worth trying to track down that book.
-tony