How would I make rounded corners for a silicon mold

Hi DIY redditors. Long time lurker, first time poster here in /DIY .

I am making a silicone cast of a farmhouse sink for my home remodel. I am trying to figure out the easiest way to create the outer and inner rounded-corners of my "positive."

For the inner-sink shape, I was thinking of using quarter-round and gluing/pin-nailing to the side pieces of the melamine board. Not sure how i would do the outside rounds.

If anyone has any tips or shortcuts, please post!

Here's an example of one of the sinks that has rounded corners: picofsink

Submitted February 26, 2015 at 11:25PM by iownslaves http://ift.tt/1LMT6VV